Just for reference, Nobuo Uematsu technically didn't leave Enix until after Final Fantasy X (though some argue saying he left after FF9 since Uematsu didn't compose all of FFX's music himself, since he brought on extra help for composing of FFX's music). That was back in 2001. So from FFXI onwards was other composers, yet Uematsu still has done some freelance work for Square Enix, he isn't officially apart of the company.
As far as FFXIII-2 goes, I'm having a really hard time getting through FFXIII cause the gameplay is so boring. I mean the story isn't bad, but its not great either (definitely not up to what I expect of Square Enix), but the gameplay is really what drags 13 through the mud. The fact that they're trying to release a sequel to 13 brings bad omens I think. However I think this is the risk that Square needs to take. Sure FFIV flunked big time and we all hoped Squeenix has learned from it, but what will be the true test to determine if the FF series has died and will save Squeenix's rep is if the next FF game they make is a big hit. I'm just questioning, why does it have to be FFXIII? There's so many FF games that potentially could use a sequel and yet they choose one that wasn't exactly the most well recieved game of their collection. Besides why does it even have to be an FF game?
Lets take the 3rd Birthday for example, much needed sequel to PE series, despite the problem of it being on the PSP. I could name several other franchises that Square has that could easily fill in and make their money back that doesn't have to be an FF game.