Are fewer books being published for RPGs across the board?
Do you mean across the industry as a whole and counting digital/POD?
If so I am very confident you are very wrong to think that there are fewer, and that the people agreeing are, with respect, not actually thinking about the industry as a whole, just individual games, and they're seemingly not even thinking very hard about those, quite frankly.
There are probably multiple times as many books coming out for RPGs now, in 2025, as there were in say, 1995, or 2005.
But the difference is they're not for a relatively small number of games which produce number of supplements, they're for a very, very large number of games, most of which produce only a single gamebook, or a small number of supplements.
We've gone from a situation where, to use made up numbers, there might be 10 popular gamelines, with an average of 50 supplements each to a situation where there are literally uncountable numbers of gamelines, but they're probably averaging about 2 supplements each, if that.
Just look on Itch.io, look on Drivethru, and realize that even those together are only a fraction of what's going on with RPGs today (probably the majority but still).
Furthermore, the average design quality of RPGs today - even one-person indie efforts - is significant higher than in the 1990s or 2000s.
Broadly, it's a shift across multiple mediums as well. You can read about the same trend in movies; there are less "mid budget" movies now, with studios putting out either high budget blockbusters or indie and/or Oscar stuff on the lower budget side. The video game market also has less stratification; you have really big AAA games and lots and lots of small indie publishers, with less in between.
Your videogame example shows how you're fundamentally wrong to assert that this means there are "less"/"fewer".
There far, far, far, far more videogames produced per year now than at any time before. It's not even remotely comparable. Most of them are indie games, sure, but indie games now are produced to higher qualities than good quality mainstream games were in the 1990s in many cases. Single programmers or tiny teams now routinely do what a team of a dozen or two dozen or more people did in the 1990s (albeit often over 1-3 years instead of six months - but a even "decent" selling indie now utterly nukes from orbit the sales of a good AAA from the 1990s - Ultima 6 sold a bit under 100k copies in the first year - there are indies that aren't even that successful that do that in a day or a week or a month or two).
Also using relative values - I'm sorry but that's just not good practice. Because values aren't relative in that way. You can't a mid-budget movie is now $70m or whatever, even if that's technically "the middle" of the budget range of movies today - that's still an insane amount of money, that's still gets you a lot, lot more than $20m did in say 1990. Also movies are a terrible model here because unlike videogames and TTRPGs, there hasn't gigantic explosion in the number of indies - unless you start counting TikTok etc. - which maybe one should? I dunno.
The shift from a bunch of mid-size products to a few larger ones and a lot of smaller ones is real - albeit not as pronounced in the RPG industry as some of you seem to think, but overall we're getting:
A) More RPGs, much more diverse RPGs (in literally in all senses of the word).
B) Much better quality RPG design than were were.
And like, I feel like even when we talk mid-size and so on, I don't think a lot people are really aware of just how much is going on. Like how many RPGs and supplements get released, get Kickstarted, and so on, every month. There are a whole game-lines out there, entire complete RPGs with many supplements that I've barely even heard of. That wasn't true in the 1990s or 2000s.
Like seriously, if you just keep paging back in top selling RPGs in Drivethru, you'll see games that are sustaining entire lines of products that you've never heard or barely heard of. It's wild. And again, I must emphasize that the quality of an RPG today is typically higher than one in the 1990s in ways except perhaps art/visual design/layout (simply because those are much more expensive, and can't be done 1-2 people usually).
I don't mean this meanly but like I am genuinely shocked at people casually agreeing with this demonstrably incorrect suggestion that we have fewer RPGs now. I expected more awareness of the sheer size, diversity, and quality of the RPG market.
Hell, ENworld's own Morrus has multiple different RPG lines. That's insane by 1990s standards! That's a major publisher by 1990s standards - and he's one of many!