The real world money to purchase those token (and anything else) come from people. Those people do not have infinite resources to spend on the game.
Ergo, if you exhaust their real world resources, and then offer them the best item in the game for $1, they wouldn't take it.
At that point, you've got the value of the total in-game economy. Because anything virtual after that would be worthless.
(Subject to demand curve, etc. But in aggregate it boils down to the same point.)
(You might be able to make the case that speculators without interest in the game would come in at that point to buy items, but I'd question how many would take the risk after the playerbase was already financially drained)
its not so clear because while your argument is true - people are only gonna spent disposable money on this - it can nonetheless be directly linked to actual transactions which happen at an incredibly frequent rate.
there is an actual cashflow happening - and not just for speculation like with cryptocurrencies - as people are actually spending the ingame currencies on their respective markets, which often removes the money from the market entirely. (not everything you buy ingame is sold by players)
and this isnt just an option, but something done extremely frequently. tokens are usually sold within one day.
(and blizzard takes ~40% more € for that token than a subscription would take, so blizzard actually has a massive profit margin there)