How about in a decade or two, when Bitcoin's market capitalization might be in the trillions, or tens of trillions? Valuable enough?
sure, but...
We're also talking about much more than 1 million bitcoins. It's 5 million+ that have exposed public keys and theoretically the entire supply if QC is capable of breaking transactions in flight.
...the greater percentage of the total BTC supply someone can steal using any exploit:
- The more BTC's market value will crash, meaning the attack's purpose changes from profit to an arson-like motive
- The more likely that a majority of previous holders reject BTC in favor of a resistant new coin, even if a fix for the exploit is discovered
The last point (ironically) resembles what's actually happening with central bank money today; people rejecting it for alternative assets because knowledgeable abusers of the system are being allowed to over-aggressively suck all the value (as well as any remaining credibility
) out of it, while the economists and policy advisers desperately try to appear to be correcting the situation 
Centralized infrastructure also requires far less coordination to secure. In a zero-day situation, governments and banks could react far more effectively than the decentralized Bitcoin network ever could. If QC broke ECDSA in the wild today, I don't think Bitcoin would ever recover.
this is very true, and so credit to the developers who have the sense to move slowly and carefully with changes/additions (even competitors to Bitcoin have behaved very responsibly, e.g. the reporting for the inflation bug, or the handling of the recent channel spoofing bug in Lightning). But we're in a virtuous circle here; very talented software developers and computer scientists were attracted to Bitcoin when it was still experimental, and now many of those same people are as motivated to contribute to furthering it's viability as they are invested. Brilliant.
