I don't think XII would work better as a spin-off since its battle system is a direct evolution of the ATB system and the game overall place much more closely to what previous FFs did than XIII. As for XV - from the very beginning, since it was announced as VS XIII, it was said that it would have an action battle system, so it made sense for it to retain that even after the change, especially since SE have been struggling with maintaining mainstream appeal after XIII. Which direction XVI will go is still up in the air, but I'm definitely way more open to varying battle systems in FF than before.