>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.