There’s two big points with your post that are unrelated but the wording makes them seem so. Your saying that later expansions invalidate prior expansions. I don’t agree with that. They certainly take you to new places but they don’t invalidate your prior accomplishments. Gear is just a token, it’s the experiences that matter.
The other point you tied in with that was a dissatisfaction with flying mounts. I hear you. I personally loving flying as a player but from a game design perspective it’s harmful to the game because it reduces player collision and thereby the potential for player interaction and socialization. Should it be in TBC? Yes.
Players that loved TBC for TBC deserve to be able to enjoy it as it was. Just like I’ve enjoyed Classic.