There's no turning back for Final Fantsay. Mainstream gaming has moved past turn based combat, everything is more action orientated. It draws in a wider audience.
If FF15 goes on to be successful and well liked then i think they will most likely keep the battle system for FF16. Having a pure, old school turn based rpg combat system is risky. I think it turns a lot of people away from a game.
At the end of the day though if the combat is engaging, dynamic and done well people will like it. The FF13 series is a good example of what not to do.