The thing that supposedly sets Bitcoin apart from other cryptocurrencies is that it's deflationary and 'immutable', in that Satoshi is gone forever and any deviation of Bitcoin from his golden idea will result in undermining its essence. If Bitcoin can get quantum-attacked then, from a technical point of view, nothing will be lost. The Bitcoin core devs can issue a word-of-god statement stating that they'll roll back the chain to before the attack, and all is well. Then they'll change the cryptography. But at that point, is it still Bitcoin? Because you've undermined the immutability. If the core devs can just say "this core property of Bitcoin is now something completely different", who's to say that they won't change their minds about the deflationary nature in the future? All credibility will be lost. Now, if you accept that, is perhaps all credibility lost already? ...