The problem with making it optional is it won't stay optional. At least not in a functional sense. The dev team isn't going to design a whole system specifically for combo consolidation only to also prune out old abilities. So for players who dislike combo consolidation, they'll eventually be forced into it as new buttons are added and none are removed.
You also list a lot of false assumptions. Button bloat has nothing whatsoever to do with tanks or healers having a single combo/button. They're intentionally designed to be simplified for perceived accessibility. What will happen is tanks will just be reduced to spamming 1111111111111 ala Healers for the vast majority of encounters.
This is a common, and frankly, irritating misrepresentation of the opposing argument. Nobody is saying 123 is "smarter" or "harder" gameplay. They're saying the feeling of moving their fingers across several keys is more engaging than simply spamming the same one endlessly. Using my own P1S run last night, if Warrior's combo were reduced to a single button, I'd have pressed the 1 key 94 times. It may not seem like much to some but for myself, that feels far more boring. Is 123 harder? Heavens no. But it's something.
And please don't argue, "they'll be able to add new abilities if they consolidation combos!" Look at Summoner. Just... look at it. They had a new slate to work with; an entirely fresh canvas. Look how many buttons it got. Do you really think they'll give Warrior two full combos or replace the 6-7 buttons Dragoon loses? I guarantee they won't.
