Maybe. I think a lot of PF fans wanted to finish their campaigns or get one last use of their books. There was likely a slow year-long trickle of people swapping game systems.
Stabilising is nice, however it doesn't imply much growth, which means diminishing sales.
For a time. Although, we don't know how much new players PF got since then. They might recovered from that loss and even surpassed the former numbers by now. Or not. We don't know, but I won't bury them before they're dead.
Roll20.
And I'm sure there are some people that got bored with 5e and switched back. But that seems to be an insignificant percentage.
Again, might be, or not. Depends on the local scene, your friends, etc. I read a lot of accounts of people going over to 5e and not wanting to go back. I also read a lot of accounts of people playing both gmaes and people finding 5e lacking and going back. I read accounts of towns and clubs where the PF games died and accounts of places where you can1t find anything other than PF. We'll see, on the long run, if PF could sustain itself, but I'm not that concerned.
We have no numbers at all for other sources, other than the sales ranks of Pathfinder books on Amazon. And the sales rank of all Pathfinder books has been tracking downward for the last couple of years, with accessories no longer selling consistently in 2012 and 2013 while the Core Rulebook started slipping down in 2014. (See camelcamelcamel.com)
And, yes, I am aware that Paizo sells through their own website so sales could be moving over there. But this seems unlikely. Because their prices are higher and their shipping is killer (and much, much slower). There's no reason an increasing percentage of people would be shopping through their site.
I might be wrong, but I think it's also like the market share percentage, because rank is in relation to other things, not the actual sold copies.
But even if they're selling less copies, I'm not surprised. The business model is vastly different that 5e's.
Again, we don't know if the PF playerbase is actually growing or if it's shrinking offline and increasing slowly on Roll20 as people look there to find games they cannot in meat space.
Yeah, agree, we don't know. Could be that, could be the former, could be the latter. Time will tell.
Even then, the problem is that the small restaurant chain in this analogy has a huge staff and needs some pretty heavy sales numbers to sustain said staff. They need their current product to keep selling well. It doesn't matter how stable their playerbase is if they're not buying new books.
But we don't know if the product still sales well, or at least well enough to sustain them. What we know is that McDonalds sells more.
I don't want Paizo to take over the world, or beat WotC. I'm not in need to make myself feel better by playing the most popular game, I'm perfectly content with the company I like the products of making those products and being okay financially. I'm okay being more niche.

If/when I do run Pathfinder again, I'm not buying more books. I stopped buying physical books a couple years back, and I haven't even picked up the last couple PDFs after realising that I hadn't even really read the prior two PDFs I had purchased. And I'll probably greatly limit the books my players can use. Maybe an Adventurer's League-esque Core Rules +1.
But that's you and you're likely more in the 5e target audience, so it's fine.
This sounds elitist as eff.
Maybe. Then, so be it.
It strongly implies that fans of 5e & WotC are not "dedicated" or that 5e's target audience was new gamers. Neither is true. The 5e playtest and design of 5e were strongly influenced and aimed at current & old players and the edition has some heavy retro vibes. It's a very nostalgic edition aimed at older gamers. That it's accessible and appeals to new players is just a perk.
(It also implies that new gamers can't be dedicated. Which is probably not remotely true either.)
I disagree. New gamers aren't just the only target audience of 5e, there are others. Old gamers being nostalgic with a game that resembles more the earlier editions' feel. People, who don't like crunch, or have issues with "bloat". People who prefer lighter rulesets, or who prefer to homebrew, people with less time, etc.
However, I'm standing by my assesment, that 5e's main audience is new gamers. The whole business plan is catered toward them, the game is catered toward them, to make picking up the game as easy as possible and I'm fairly sure they are making up the biggest demographic of 5e players right now.
Also, change the word "dedicated" to "long term fans, who'd like to see more content" (mechanical or setting/fluff)" and I think we're okay.
And yeah, "dedicated" to me means, that you're interested in this thing enough, that you spend significant time with it, want to read more of it, try more things out, etc. That you want more stuff, plain and simple. WotC just doesn't have enought stuff for 5e to satisfy those, especially if you want official material. So, I think those people will, after a time, converge toward other games.
Again, I'm not saying 5e doesn't have "dedicated" fans, in the sense of dedicated meaning "long-term" for another reasons, but I do think that it is a minority, while new and casual gamers are the majority. And no, I don't thin "casual" is a demeaning word, but it's a different level of interest. I also don't think that being aimed primarily at new gamers is a bad thing. It's just thing.
They're close. It's probably a good time for them to switch to one adventure per year and have other types of product. Which, conveniently, appears to be what they're doing.
We'll see how they will line it up with their "anti-bloat" approach. If we'll see actual setting content, for example, outside of the APs, well, I'd applaud that.
The problem is that you don't make a new character every four months, which is the rate new hardcover books come out.
Each of the two APs I ran took up the better part of a year, and only had a single character change during them. During a play-through, three books came out with new options plus innumerable Player Companions. By the time my players could play a new character, the concept they had from reading one book had been replaced by one from the most recent book.
You mention wanting to play a shifter. And maybe you will. Assuming you make that new character before Planar Adventures comes out.
Collectors are a nice audience, but they're a minority. And there's only so much content even collectors will buy before they realise they have five or six books they've never used and a couple they've barely even read. I don't think Ultimate Combat has ever been used at my table. Or Occult Adventures. Let alone Mythic Adventures.
Plus... if you're a collector buying books just to collect and read and not use, then they don't actually need to be for a game system you're actually playing. They could be art books or map books and it would still count. Or other systems. When I decided my PF collection was "good enough" I moved to Shadows of Esteren. And filling in some gaps in my 2e book collection.
Advanced Class Guide is a great example. They had ten new base classes and not nearly enough time to remotely test them all. So the entire book is full of shaky mechanics. Rather than, say, pitch the classes and playtest the materials, then pick the six favourites and be able to adequately test and balance them all, they included all ten because they decided in advance that was the contents. Whether they were ready or not, or whether the fanbase really wanted them or not. It was pure content for the sake of content.
(And while you say you liked at least half of them... how many have you actually played for a significant time? Because, liking five classes is enough content for five full campaigns, or 2-4 years of play. It's literally the only PF book you needed since it was released.)
The vigilante is also a great example of content for the sake of releasing content. It exists solely because the other Ultimate XXX books had a new base class. It could have been a series of archetypes or a feat. Or a 1 to 5 level prestige class. (Which makes the most sense as being a superhero is something you train into.)
In short, your whole thought process is rather alien to me. I'm not measuring content by how much of it I actually used. Given, that's only a small portion of it. But that's not a problem, because having cool stuff to choose from (mechanically) and stuff to read and being inspired by (fluff) is a merit on itself in my eyes. Having options is a merit on itself. If I'd play only 1 class from those in ACG, that's fine.
But hey, I'm the kind of people, who like options and detailed settings and yes, even metaplot, so there's that. I never got into the mindset of not liking, for example FR, or WoD, because thre are too many stuff for it. Well, duh, that's the point.
I won't say I'm a collector, I'm not buying books just to sit on the shelves, unread. I buy what interests me, but if something interests me, I'd at least read it, even if I won't using much of it and that's okay. RPG books aren't just tools for me, but sources of inspiration and leisurely reading too.
Pathfinder is probably the most supported game system currently published. Possibly second, behind RIFTs. There are more feats, more class variants, more spells, and more magic items then there was for 3e. A couple years back and Paizo was putting out more content than TSR did at its publishing height.
The question is, what percentage of content published by Paizo do you think has actually been used? How many of the 2500 feats have actually been used? Or the 400 subclasses? Or the 40-odd PC races?
How many options have never been taken once in a single game?
I'm fairly sure everything, or elmost gotten used by someone somewhere, along the years. That's enough. Nobody will count it. They won't get more money if more people will try more of the options. It's not like Youtube, where you'll get money by more hours watched. If someone bought a book, enjoyed reading it and at the end only used one new class, or one location from it in their entire gaming career, but considered it a good read, that's enough.
It also means you'll get to have more niche options and that is appreciated.
I quite like them as well. There's some great stuff in a lot of those books.
Yeah, as I said, my favorites. I'm msotly a fluff-centered guy, truth be told. Reading crunch bores me to death, but I like to have it for character options.
Here's the thing... do they need to be regular? Making them monthly or bi-monthly just means they have to release one whether it's ready or not. What if they just released one every 2-4 years when they have a really good idea and it's done? When they need to tie something into the AP.
A hard schedule worked for the subscriptions... but that's magazine thinking being applied to books. It's needless. Again, how many of the 87 Player Companions existed because they needed to release a book that month?
No, I don't think it would have to be regular. It could work irregularly or as thematic tie-ins, as you said.