>If the US and China crack down on trading houses and currency conversion (aka ban them) there's not enough mining happening elsewhere in the world to keep the chain safe.

Getting such a ban in place would be the hard part. If US banned bitcoin, I'd imagine US's enemies would sponsor bitcoin exchanges/miners just to piss off the US. Also, considering that miners be located literally anywhere and only need a modest amount of bandwidth to operate, shutting down clandestine mining operations would be very difficult. See: all the illegal drug labs/grow ops around the world.

On the contrary, if it ever shrank to an extent that posed an existential threat I think it is almost certain that a different consensus algorithm (such as proof of stake) would be adopted.

There's no telling how the legalities surrounding crypto currency will evolve but I seriously doubt that it will ever disappear.

This happened - a mining group approached (and possibly passed) 51%, so they directed some of their miners to other pools because they didn't want to devalue their earnings.

Anyone with that much mining power has a huge financial incentive to keep the golden goose alive.

Even if you ban legitimate usage of cryptocurrencies, it will still have value underground for criminal activities as a settlement mechanism between criminals (Bitcoin/Monero seems easier to move than bags of physical cash).