"Traditionally" is a bit too strong of a word to use, as there has been only a single instance in the main-series games with an single-gender cast, the aforementioned FFX-2 (I've not played many of the spin-offs, but I seriously doubt it's a different case there).
A precedent is there for sure, but it isn't a pattern; single-gender casts are not at all a regular thing in Final Fantasy. It's pretty much just a way to show to those Sarkeesian-esque types that Square has absolutely never favored one gender or the other when it comes to the vast majority of their Final Fantasy games, with the long exception of an all-female cast having a story driven reason behind it (Two of the four main male characters die at the end of FFX, the others become leaders/fathers of their respective villages).
As for FFXV inevitably going to be criticized due to today's gaming climate, I agree completely. It's just the way the industry works right now, unfortunately. I feel like the best way to handle it is for the true fans of the series to just point out the hypocrisy and move one.
