Don't worry about Bitcoin, a small niche. The world will have BIGGER problems to worry about with the arrival of quantum computers. Hahaha.
The world will be able to fix the quantum issue and implement a quantum secure mode through rewinding, freezing, correcting accounts.
But Bitcoin can't and that will lead to forks.
I'm confident that the community will come into consensus to hard fork, if the threat of quantum computers will be the "death of Bitcoin". It will not be close to a debate.