> would require all holders (not miners) to actively update their wallets. Basically infeasible.
It doesn't require all holders to update their wallets. Some people would fail to do so and lose their money. That doesn't mean the rest of the network can't do anything to save themselves. Most people use hosted wallets like Coinbase these days anyway, and Coinbase would certainly be on top of things.
Also, you don't need to break ECDSA to break BTC. You could also do it by breaking mining. The block header has a 32-bit nonce at the very end. My brain is too smooth to know how realistic this actually is, but perhaps someone could do use a QC to perform the final step of SHA-256 on all 2^32 possible values of the nonce at once, giving them an insurmountable advantage in mining. If only a single party has that advantage, it breaks the Nash equilibrium.
But if multiple parties have that advantage, I suppose BTC could survive until someone breaks ECDSA. All those mining ASICs would become worthless, though.
Sometimes there is no valid hash found for any nonces in the 2^32 space and the timestamp and/or the extra nonce in the coinbase transaction in the block header have to be updated and tried again, so at least it's not quite that simple (simple, as distinct from easy).
It doesn't require all holders to update their wallets. Some people would fail to do so and lose their money. That doesn't mean the rest of the network can't do anything to save themselves. Most people use hosted wallets like Coinbase these days anyway, and Coinbase would certainly be on top of things.
Also, you don't need to break ECDSA to break BTC. You could also do it by breaking mining. The block header has a 32-bit nonce at the very end. My brain is too smooth to know how realistic this actually is, but perhaps someone could do use a QC to perform the final step of SHA-256 on all 2^32 possible values of the nonce at once, giving them an insurmountable advantage in mining. If only a single party has that advantage, it breaks the Nash equilibrium.
But if multiple parties have that advantage, I suppose BTC could survive until someone breaks ECDSA. All those mining ASICs would become worthless, though.