The spinoffs and so forth is an excellent way for them to cut creative costs. In addition, the global world has proven that there is an appetite for franchise characters, spinoff stories and merchandise.
Arguably, the name "Final Fantasy" is one of the most valuable brand names in the video game industry right now.
Also, they will probably use Final Fantasy XIII-mania to kickstart interest in Final Fantasy XIV, which is where they can make real year-over-year profits.
Every year, they say that they'll never release a normal Final Fantasy game ever again... but then we saw X-2, XII, and now XIII, in addition to all of the FF VII spinoffs