The fact is that none of the post-XII FF titles, other than FFXIV, have done particularly well. The franchise does not have the same reputation it used to have, and the only reason XIV is keeping the company alive is because it heavily leans on referencing the glory days of the franchise when the games were just a lot better (most of SE's other money makers were similar nostalgic crossovers like Dissidia, Opera Omnia, and KH).
There are two ways where the franchise is failing. The first is simply failing to deliver on the broad world-building of prior titles. I think XVI did better than XII, XIII, or XV in trying to provide other things to do than hunt quests as side content, but the franchise still isn't at the level it used to be. I would also postulate that more recent titles simply haven't done as good a job in making distinct, fun character archetypes to play with. While I can see why the franchise may have felt the need to try to grow more adult/realistic beyond these features to compete with a very saturated JRPG market that followed in its wake; I still feel like these features were what made FF a superlative experience and they could still be captured in a more modern way without being totally discarded for the comparative blandness we got in XII, XIII, and XV.
The second is that the stories just have not been as thematically deep as they were in VI, VII, VIII, and X. The golden age was really about taking the fantasy setting to really delve into some impressively philosophical topics for video games, and we just haven't seen that in recent games. They have kind of settled back on safe, tropey fantasy plot structures.
More to that point, I think the most egregious problem with the writing is the increased insistence on retrodding the "six summons" canon as increasingly central features of the plot. Summons have slowly been swallowing FF worldbuilding whole, and frankly that whole concept was already played out in XI. The reason why none of these titles have been home runs is because we have *already* explored the Ifrit/Shiva/Ramuh/Titan/Garuda/Leviathan thing in XI, and again in XIV. It is why both XV and XVI weren't totally mind-blowing to anyone because so much of the framework was familiar. The franchise really needs to step away from the summon canon. It needs to write stories again that weren't so focused on summons. And it would heavily benefit from introducing fresh blood into the summon roster like we got with Quetzacoatl, Eden, Valefor, Ixion, Anima, Brynhidlr, and Hecatoncheir.
If they want this franchise to return to being the gold standard of the genre, they need to stop safely recycling stuff and come up with new ideas again instead of rehashing old ideas. And they need to so soon because XIV is almost out of things to steal from the golden era games. I'm honestly stunned SE haven't realized this yet.