@cliff : If they were to change the POW, Core would fork themselves off.
The miners could just maintain the Core software from prior to the fork themselves. As long as they maintain a sufficient "old-style Core" node network (pretty easy / low-cost) they could easily carry on their operations.
Only incentive to switch to a different (non-Core) fork would be for the features it might offer.
However, if the miners go with a pre-SW version and just up the block size, they could very well negate much of the public draw that current hard forks offer, while retaining their prime positions in the food chain, and making a "small block" Core fork look pretty uninteresting.
That said, the market is not oblivious of the risks of having most of the mining and related industries in one country, and if events like you sketched should ever come to pass, I'd guess Bitcoin would in for some extreme price turbulence, and then it's anyone's guess what might happen to its current miners.