The main problem is probably not overall quality of Final Fantasy's anymore. You yourself just said you loved Final Fantasy XII, the Final Fantasy that came immediately before XIII. They're big problem is the time it takes to develop a new one. XIII has fans, but if you don't like it you are likely to wait 7 years for a new one, making the company seem unable to produce games anymore, because you had a sour taste in the last one you waited 7 years for. But, in single player experiences you've enjoyed, you only seem to be disappointed with their newest addition, not even the latest two.