Personally I love the concept in classic wow, that if you want something pretty, a nice set or a nice looking unique mount you have to earn it via playing in game and not being richer in the real world.
As far as I am concerned, this one thing of bringing paid for cosmetic mounts etc into wow classic TBC, Blizzard bring in won’t put me off, but if its the start of more of this type of thing where we are leading towards a retail style game then I am going back to classic.
If Blizzard bring those changes into classic as well in greater quantity and classic starts to look and feel more like retail then I would stop playing wow again just like I did with Mists of Pandas came out. I have been through this slippery slope before and been burnt. Thankfully we are not there yet although there are some warning signs.
As far as I am concerned, cosmetic look is a central part of the game reward from in game activities along with game play benefits as well(dps etc). If I want a cool looking tier 2 outfit for the warrior, I will need to raid until I get it, my reward: a cool looking set of armour that I earned in game.