This isn't as simple of a change as it may appear.
You can't just slap in Auto Attack and rip out the Stamina Gauge without restructuring how stats work, how enemies behave, the effectiveness of certain attacks, and pretty much everything that has to do with the battle system. Once that's done, they can move on to adding content that will complement the system. There's a ton of math and planning involved and I'd rather they not rush it.