I think the quality of the music in Final Fantasy games took a hit in FF X and XI. Then XII happened and it was different. Took a bit longer to fully appreciate the change of style, but once I got over the lack of Uematsu's ultra catchy tunes, it grew on me. Special mentions: Seeking Power, Symphonic Poem Hope, Loop Demo, one of the best version of Clash on the Big Bridge, The Phon Coast, The Sohen Cave Palace, to name only the first that come to mind. The game even started with the classic Final Fantasy Theme in full glory.
Then XII came and it was ok to good. Some memorable theme, like the Sunleth Waterscape and Blinded by Light. But overall, I thought the themes didn't stick to my mind. They did a good job at setting the proper moods, but weren't themes I'd whistle like the ones from prior games.
Overall, I wouldn't say the music "collapsed" the Final Fantasy franchise. There are much [MUCH] bigger problems
than this that S-E needs to fix for the next games. Like proper exploration, a come back of the world map and controllable airships/transports, and stories that are actually not for early teen. FF VI, VII and to some extant VIII and XII achieved that pretty well, even if XII lacked emotional impact... IX was also a nice nod to the classic elements of the franchise, but the story wasn't really anything we hadn't seen before...
But I digress. I think the music has remained of pretty high quality across the board, with the exception of X and XIII which I felt weren't completely consistent throughout, but the change of style from Uematsu to his followers certainly needed an adaptation for the listeners. And since most of us were used to his style and enjoyed it ever so greatly (with good reason), the passage from him to the others was greatly felt.
Of course, you must then add the power of nostalgia to the mix...
