In terms of Square's mentality

FFI-FFIII: We might have something going on here...

FFIV-FFVI: Guys, we rock!

FFVII: Guys, we nailed it.

FFVIII-FFX: Not as successful as FFVII, but each installment has its own dedicated fanbase.

FFX-2, Compilation, After Years: Whoa guys, we can exploit fan nostalgia!

FFXII: Well, this game was good. Now time to make Ivalice games with none of the political drama that made the first three so good (I'm counting Vagrant Story)!

FFXIII: OHMYGHERD LEHRTNING! LOVE HER. WORSHIP HER! IT'S THE SECOND COMING OF CLOUD STRIFE!

And then there's the MMO's, and assorted spinoffs.