Blizzard isn't going to charge extra for WoW:Classic, it'll be something they just give for the players to check out and attract the private server player base back into subscribing. They won't attract anybody to play it if they make any of those QoL changes. Defeats the whole purpose of having a Vanilla server if you make any gameplay changes. No achievements, no mount tab, no personal loot, no graphical changes, no aoe looting, none of that because that isn't Vanilla anymore. They are going after a particular fan base and any changes to that gameplay, they'll lose the entire crowd they are going after.
This isn't a billion dollar idea they are coming up with, it's something cool they are creating to give back to the players it isn't meant to be a money grab besides bring back subscription they lost because people hate the current state of the game.