I think it's important for people to realize that the mainline games have been pretty well-received critically overall even with their flaws (FFXV got a lot of good critical buzz around release despite its issues!), not to mention that there's a bit of a cycle with the series. Every new game is the worst thing ever, a sign of doom for the series, until we get a new release or two and then it's all rose-colored glasses. Obviously I don't know if it's the specific same fans, but I remember how much a lot of fans hated FFXIII on release, yet I see a lot more appreciation for it nowadays, as an example.
I do have a hard time thinking the series is on a decline overall, tbh. The series (and more importantly, the entire industry) was in a very different place back in the PS1 days, from how we interact with stories in gaming to our standards of storytelling and the differences that things like voice acting bring in. Rebirth may be selling less, but that's to be expected: it's a PS5 exclusive when the last game was on PS4 and PC, which have a way larger install base. It's a direct sequel, so people might either wait for the trilogy to be done or decide there's too big a barrier to entry (or they didn't like Remake). But with critics and with player ratings, Rebirth got a lot of positive reception. Sales aren't everything.
Uh, I'm so sorry if this sounds rude, but it's literally critically acclaimed. Like, by critics. That's what the phrase means: acclaimed by people who critique media professionally. There's some leeway - maybe its flaws hamper the experience way more for you than others! - but there's a reason Rebirth has such high ratings, whether it's OpenCritic or MetaCritic or what-have-you. Doesn't mean it's perfect (hell, a lot of the glowing reviews do mention the pacing issues, for instance), but from large gaming-specific outlets (IGN and Destructoid) to general media criticism (Guardian or Washington Post) and smaller, more discerning critics (Jimquisition, Paste), it's been well-received.
Gaming is one of those things where yes, sales are important a lot of the time, but they're not the only metric to gauge something's success. If they were, Square would have completely folded after The Spirits Within.