If a cartel of miners tried to block all transactions, there would be code for a hard fork within 3 hours making some trivial change to the PoW that rendered the current mining industry worthless and the hard fork itself would probably happen within a few days. And then life would continue on as usual, except that the next set of miners would know how suicidal such a move would be (and on a more contemplative schedule further changes would be made).
So, according to that post, the cartel cannot force the users to do a compatible change the protocol because the users who do not want any change to the original protocol would immediately agree to make an incompatible change to the protocol. Makes sense...
That hard fork would of course prevent *all* existing miners, including the faithful ones, from working on the 'born-again orthodox' chain. That chain would be no different from any other junk altcoin with a tiny CPU-based network. The cartel probably would have enough money to rent cloud computing and jam that rebel bitcoin chain too. Would users choose to run that risk?
Even if the rebel fork survives, it will in time see its mining get centralized again. That is, users may escape from *that* cartel, but not from the centralization of mining itself.
If the rebels opt for a hard fork, exchanges and payment processors would still have to upgrade their software anyway, only they would have to choose between the cartel's chain or the rebel chain. Which one would seem to be a safer bet?
Among the users who are not willing to switch to the cartel, many will not upgrade to the rebel version right away; instead they will just stop using bitcoin until the winner has emerged. And the rebels could always defect the ranks at any time, with no penalty or risk, and they will find their coins still there in the cartel's fork,
minus any possibly even most of the transactions that they made since the fork.
Only ideologically motivated bitcoiners would be bothered by a proposed change in the protocol like the one I outlined in my reddit post. How many of those are there? The selfish bitcoiners will care more about their wealth (current or expected) than saving the world from greedy bankers and fascist government. Which of the two chains would seem safer in that regard?
If the cartel's change is not too radical, the holders of large hoards of cheap coins will not want to sell, for the same reason that they are not selling now: they would still hope to make large profits in the future. On the contrary, they would hold and support the cartel's decision, in order to preserve that expected return.
EDIT: the cartel would probably mirror the transactions of the rebel chain on their chain too, as long as they are valid in both chains.