In cryptography, one of the premises, along with not trusting third parties, is to always assume the worst-case scenario.

In Bitcoin, this has been ignored for years.

The worst-case scenario is that a government or private entity secretly builds a sufficiently powerful quantum computer.

In fact, this is the most plausible scenario because they would have an enormous tactical and competitive advantage. They would have no interest in making it public because they could spy on all their opponents and could also destroy all confidence in Bitcoin in a matter of minutes.