You do forget, the coins have usually 2 sides:
The Wow market, is on first place - market, so the common rules work here too: tokens could inflate enough at some point, so the donors would see not enough profit for them to sell them ingame, but buying gold directly.
From my humble goldselling experience, the most buyers need the gold instant, either because its impulse buying for them, or just because they really need them to buy something very soon, so the token+cashout on AH could not be fast enough for them. They already know, that gold buying would not lead them banned, so they do not care for the consequences for not using the "legit" way of obtaining gold.
Then - the back side of the coin:
Technically legalizing on the gold selling for Blizzard could harm their pockets more than they had expected, even in multiple ways, I can brainstorm two ways right away:
There is a stable playerbase in WoW, which do play it, only because of the competitive environment, without the P2W factor! A lot of them could simply leave the scene without questions ...
There will be definitely flood of scammers, selling tokens, obtained with CC fraud methods for gold, so this will lead to tons of problems not only to the Blizzard Customer Support (The Game Masters), but the forums will eXplode!
Of course there are much more problems, this tokens could drag to the Wow scene, but only future will prove it!
And one last point - the tokens will be sold for GOLD, and someone have to make this gold from nowhere, guess who will be faster with the goldfarming?