I came across this thread with the same question as OP. I had this question because I was debating the value of spending the time to farm a particular mount in classic/tbc if I could only ever use it in classic/tbc. (By the way, I decided it wasn’t worth farming it, and so am less motivated to play, fwiw.)
However, I would say give it time. Don’t underestimate Blizzard’s ability to undo things they originally claimed were set in stone, for the sake of monitization. The list could go on and on of examples of these over the history of the game.
My theory: Once classic/tbc/wotlk expansions run their course, all the folks with hard stances about these issues will mostly be done playing. They’ll need a way to continue to money train, and so they’ll add some classic-to-retail linking to get the subscriptions up.
An example of what’s coming: I purchased the TBC deluxe edition, and I got the little promo mount both in TBC and in Retail.
I know the argument will be that it was a purchase and not something I did in the game, but like I said, it’s just a matter of time.
I agree they wont ever allow retail items to come into classic/tbc, but I think they’ll continue to soften up the rules for the other direction, because like the other person said, it doesn’t actually impact anyone playing classic/tbc whatsoever, and it will lead to more people playing to round out their retail account with things they missed out on. And this = subscriptions = money = the motivation behind everything blizzard does. And bacause of this ultimate motivation, the people that would be “mad” because their achievements in retail vanilla will be “deminished”, it wont really matter because those people will no longer be active subscribers anyway, since the classic/tbc/wrath re-releases will have already run their course. In other words, log into a vanilla classic server now that TBC classic is out, and it is an absolute ghost town. The same will happen to TBC classic when WOTLK classic comes out, and likewise with wrath runs it course, the same will happen there. Most of those folks will have their subscriptions lapse at that point. And then blizzard will open the gates.
Sorry for the long reply, and for reviving an old thread. lol.