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Basically it assumes that exchanges will take the whole market hostage. This is very similar to the theoretical idea that in bitcoin the big mining pools will take the mining hostage and only build on top of each others blocks and censor small miners.
Correct, mining could theoretically become a shitshow. And it would only take a conspiracy of a few pools to have 90% hashing power.
But don't forget that pools consist of individual miners that would sanction this kind of centralization because it is in their own self interest.