So literally the same as every monetary policy change involving every currency that didn't use PoW in history...
This has happened multiple times with attempted hard forks of Bitcoin which have failed because once you change the monetary policy once, the promise of hard money effect disappears. So the original monetary policy remains in place and the original network continues as the reigning champion.
If this happens I can technically stay on the original protocol, but that would be rather pointless if a sufficient majority abandons it.
The only real problem with that is that with a small hash rate, bitcoin can be attacked more easily.
If bitcoin is the monetary backbone for many nations, they will subsidize miners to maintain the balance of power. That is the actual scenario that I'm optimistically predicting.
If bitcoin isn't the monetary backbone for many nations, by then, then it's probably a failure, and should probably be allowed to die.
It's also very possible that transactions fees alone actually will be sufficient to support a high enough amount of hash power to secure the network.
Still plenty of scaling left in the Bitcoin ecosystem.