I will say Roll20 and Amazon really might not be reliable metrics for this, over on the sub no one mentions roll20 when we're asked for VTT recommendations, and similarly a lot of people are probably buying direct from Paizo depending on Amazon price shenanigans, the demand for pdfs, and such. I get that they seem like the reliable metrics on the face of it, but Roll20 took hits from that whole controversy, Paizo's market is probably doing a little more research on which VTT they're using too since the product is aimed a little more at power users while still being accessible.
There's a lot of speculation here based on your over reliance of the subreddit as your source of information. As someone who has to look at survey data for a living, relying on message boards is very problematic. It's a piece of information, like any social media site, but it can't be used reliably by itself. A poster is generally more enthusiastic about a topic than a lot of members of the the public in that they voluntarily register and share their opinions. It's useful to see this positive indicator, but can't be used as the sole indicator of a system's health. I'd make an argument if that was the only data point we have. However, that's why you can't disregard Roll20 or Amazon sales rank, which with estimators out there can give you a part of the sales numbers.
I agree with you that using Roll20 user statistics by itself as a strong indicator of Pathfinder interest or lack thereof is problematic, especially if people are leaving for other VTT. But, we have to look at why people are leaving. Is it because people are leaving en masse? Nope, else you wouldn't see subscriber numbers up this quarter. Probably it's because of the lack of Roll20's support. I think Pathfinder is in a death spiral on that Roll20 where lack of support leads to lower number of games as as people leave, which leads to less support, etc. I don't think the death spiral is a knock on the game itself, but I'm not encouraged when I wonder why Roll20 didn't put the resources in from the beginning. Maybe someone better in the know can explain.
If anything I would use the growth of the subreddit to better capture the growth of interest in the system. Their growth has only really accelerated, which to me is pretty convincing evidence that the game as a whole is growing pretty aggressively-- more users are looking for resources and discussion on the game, regardless of what they're playing on, or where they're buying their books. Even though thats only a fraction of the market, I think it probably speaks to the greater volume of players that they represent a fraction of.
To me, this looks like a cherry picked stat to fit your conclusion of growth. It's a good sign for the strength of the game, but it's only one good sign. I've not yet seen signs of greater volumes of players elsewhere on other roleplaying message boards. What I'd expect if there were overall good signs of growth is message boards beyond Redditt and the Paizo's board where there is robust Pathfinder 2e discussions. I'd also expect more YouTube streams and podcasts of campaigns or discussions. I'm not seeing that either when I look for it, which could just be because of the fact they aren't getting a lot of hits.
I do expect when a game like 5e is over 6 years old you are going to see people leave it to play other games. This churn is part of a games lifecycles. If a game is healthy it gains new players faster than it loses them. That people are leaving 5E and going to Pathfinder 2e is again a good thing, but is it enough? There was that recent high profile Taking 20 "I'm leaving Pathfinder" vid from someone who initially rated Pathfinder 2E better than 5E. Is that an indication of people leaving the game faster than people coming in? Way too early to tell there, but the positive subreddit stat should be taken with a big grain of salt.
I'd actually argue the drop in Amazon sales might have more to do with that massive humble bundle deal they did a little while back, it literally cleaned out their entire stock of Core Rulebooks, it might have front loaded a lot of the interest and channeled it away from Amazon.
That explains a dip for a period back in July 2020 and a bit afterward, but why would it still be a factor seven months later? I don't think so. The current sales rank levels in the 11,000s - and one day this week as low as the 15,000s, which I think was a short blip and not a long-term bottom - is coming at a time when there is plenty of supply on Amazon and has been for a while. I agree the humble bundle probably front loaded for a time, but not now and that argument is no longer valid. I'm even less encouraged for Pathfinder 2e's future when I see that old camel camel camel that shows 2e's current trend is already looking like 1e's trend in late 2017, and that game had a constant several years of high Amazon sales.
I have no doubt that Paizo overall health, even as I remember how many of the signs people point to as reasons for Paizo's health could have been said of TSR before the WOTC acquisition. I actually hope the game does well for the next few years. My philosophy is that whatever keeps people gaming is good, and the industry needs multiple strong games. However, sixteen months after it's release, I'm not seeing enough indications of a game with long legs. With new data, I'd be happy to change that position.