This was already pretty well hashed out (heh) during the 'core'/'cash' issue when there was an attempt to fork in an expanded the block size. Both chains still exist. Bitcoin operation is entirely up to the miners to determine the heaviest chain, and that's like two entities (the number of entities required is called the Nakamoto coefficient). It's not magic, but there is a huge cult built up around it by scammers, rubes, opportunists and speculators.

Miners enforce the consensus rules but they can't change them. If miners try to change the rules, exchanges have no obligation to follow.