My guess is that Square thinks that an action game has more chances of selling better than a turn-based game. I think they may have the idea that in spite of how good a turn-based RPG can still sell, an Action RPG could overcome a hypothetical sells cap that a turn-based RPG can't.
I am not going to defend that argument, I just think this is what may be in their heads when they decided this gameplay approach. And/or maybe they just simply wanted to make those games action RPGs? Could be possible.
I agree with your argument but when ff7 came out most gamers in the US didn’t care about rpg but the game was so spectacular that everyone ended buying it anyway, if square delivers a quality product it will sell regardless of the gameplay style, besides most people disliked the ff15 combat so what’s the point of keeping it?