I would hesitate to draw conclusions based on those games. In both scenarios, the second entry was significantly better than the first (in most people's opinions), and the third game was a step down. There is likely to be more drop off the more parts you do, but I wonder if since they'll be reusing a lot of assets and such, the later entries may still be profitable even with declines in sales.
Well work on the sequel is already in process. Its possible they will resuse some assets and character models. Although I doubt it because the next step is PS5 and they will probably need to create a lot of new stuff for it, and now new gameplay mechanics for characters like Red XIII, Cid etc. Fortunately it seems Square are using Unreal Engine now so that seems a lot eaiser to devleop for than Luminous and I think we can expect the next game to be even better.
You know when you use the ATB and the game goes into slow motion and the explosive fights at the end. If they keep going they way they are I have no doubt the next game will be better, in the same way like you said about Mass Effect 2, Assassin's Creed 2, Arkham City. I think they will knock it out of the park for the next one.