I dont think it matters too much.

By the time the next installment comes along I won't care about my level 50 characters.

I imagine most of us will replay the first installment just before the second one is released, but I imagine the majority outside of FF forums will not.

Someone who is not a huge fan who maxes out characters isn't going to appreciate a difficulty spike as soon as they start playing a "new" game, just doesn't make sense.

I imagine they would offer some kind of carryover of things like summons or even offer them as DLC or something.