I didn't say Bitcoin will be safe without upgrading. I'm just saying it's not sensible to imagine that Bitcoin will not be upgraded well ahead of time, in order to deal with quantum. The topic of quantum keeps coming up repeatedly in the Bitcoin community, and everyone is well aware of its status and progress and potential threat. It will be addressed, if & when needed, according to the market's demand. I have faith in making that assumption.
In the whitepaper mthcl showed that PoW blockchains are inherently vulnerable to QCs because they do hashing at quadratic speed. So you think that miners will agree to throw away their ASICs? I predict a lot of drama, much more than all dramas that have happened before...
Wouldn't miners just all be competing with quantum miners? I don't see how that would be any different then them competing with asics...