But that's assuming that the amount of enemies and length of dungeons will be the same as it is in the past titles.
But you don't even need to have the same number of ennemies.
Just get any random battle, say, 4vs4: even if you can kill ennemies much faster than before (which would drastically change the feeling of the game), you will STILL be forced to see multiple attack animations before the second turn... where you might have to use support and healing, with their own animations. Which is normally not needed in combat, it's just a click in your inventory.
And of course, if you just remove entirely the many situations where you fight 20+ ennemies, it's a huge blow to the flavor of the franchise.
You can't be surrounded by ennemies, now even if you kill 5 ennemies a turn, you will still stand still for 15 more animations before the next turn. Which will not only look stupid, but be a complete bore.
Slapping a turn based system in any action game mens extra animations, extra busywork in support, and a completely different combat rhythm.
And all of that is mostly one thing: time wasted doing nothing. It's not like those were already 100+ games with an absurd amount of repetition.