I think that it is very likely that, sooner or later, the miners will use their power to force changes to the protocol. 

It will not be totally trivial, but it will be much easier and quicker than the BIP66 or BIP100/BIP101 changes.  There would be no lengthy discussion on forums, blockchain voting, etc.  They cartel will just announce their decision to change the protocol with a couple of months in advance, with a suitable "for the good of bitcoin" spin.  They will put up the modified programs for download, that trigger a hard fork at a specified block number.  They will warn everybody that those who fail upgrade before the deadline will be unable to use their coins until they do.

Knowing that the cartel means what it says, most clients, nodes, services, and non-cartel miners will upgrade before the fork, and will sail smoothly through it.   The cartel will use their hashpower to kill the old branch, if some recalcitrant miners will insist on mining it; then any clients still running the old version will be unable to move their coins.  Those laggards will either upgrade (and find their coins still where they left them) or lose their coins.  And that will be it.


Indeed, I am convinced that most bitcoiners will not care about a protocol change imposed by a mining cartel, if it does not affect them directly.  In fact they will support the cartel and pretend that they approve the change, to preserve the value of their coin.  I am pretty certain that you will be among them.  If you don't have a problem with 5 pools having 70% of the power, or with the 5$ cost of your transaction being subsidized by investors, or with the growing pyramid of "debt" -- then you surely will not have a problem with a mere postponement of the next halving, or with the extinction of independent relay nodes, for example.