I haven’t explained it, because this is common knowledge that we have known about since Wrath of the Lich King.
The more universally applicable or “useful” a quality of life improvement is, the more “required” it makes people feel like it is. No one needs most of the things that are quality of life improvements… but they are quality of life improvements. Are you going to argue for its removal, even if you later on discover the improvements made the thing worse?
That’s the inherent danger of them, and we can use a nearly endless number of real world examples. Everything from stores doing away with fake “sales” tags going almost bankrupt, to how cars cause something called “induced demand” which essentially state that the better you make it to drive - the worse it gets.
Quality of life improvements are only improvements if they actually improve on something which was a problem. Repair vendors on mounts sorta did this, but also not. Because they just put more repair vendors into the game, so the real strength of it became that you could sell anything anywhere at any time.
So what problem does an auction house vendor on a mount actually solve, and how does it make things better than just going to a major city ever since they started putting non-engineering auction houses into expansion capitals?
There isn’t a specific vague problem that I anticipate because… it is an improvement, but… to what? This is the exact same issue that Waterstriders caused back when Mists of Pandaria released. Why use a different mount when that’s the only one that’s useful to you?
This is also why I’m less concerned now because of how flying has reshaped how we traverse the world, and as I mentioned they started putting more useful NPCs around the world more easily accessible to us. But there are other issues. A bizarre one is that Blizzard had a precedence before that EVERY store mount could fly.
Now not all of them does. It would be beyond silly to argue the brontosaurus should… but it means that the mount has broken a precedence. Another one is that you can now literally buy utility in the store, which you haven’t been able to do before.
This one mount violates EVERYTHING we know about how people interact with convenience as well as it shattering several precedences that shouldn’t ever be broken.
So… when I say that this is likely not game killing, it likely isn’t. But it is without a shadow of a doubt objectively HORRENDOUS for the game’s overall health. It is time limited, and maybe the world has changed enough where this isn’t that detrimental… but it is still, for the sake of the game’s overall health, the worst item that they have ever sold on any of their stores (that relate to World of Warcraft) in the history of World of Warcraft.
Most folks on these forums are just going to respond to a meme when pointing out these things. That’s why I haven’t mentioned it, because most folks don’t care about understanding game design, human psychology, or the paradoxical nature of conveniences.
But you asked so… here you go.