People don't upgrade their nodes because they are compatible with the network, if there was a hardfork, you can expect them to switch rather quickly. Look at ETH or other alts, they have hard forks almost every year and they go pretty smooth.
a hard fork is not required to implement a post-quantum signature scheme. (that's probably a good thing since bitcoin isn't some centralized shitcoin---BCH and BTG are what happens when people try to hard fork it)
what are you saying---that we can literally just wait until ECDSA is broken and then do an emergency hard fork? that won't work. by then it's too late: QC could possibly break transactions in-flight, meaning the entire bitcoin supply is at risk of theft---outputs being moved to quantum safe addresses could be stolen in that scenario. even if QC weren't fast enough to do that,
there would be 5+ million coins ready for the taking on day 0. are you considering the potential consequences of that?
If quantum computers became real today, the world would be in a lot of trouble - bank account, emails, websites, military communications, classified data and so on - but you don't see them proactively moving to post-quantum crypto right now
centralized organizations can implement new encryption standards at the drop of a dime. bitcoin cannot do that because it's decentralized. and as outlined above, its inability to do so puts the entire monetary base at risk.
And cryptographers usually retire algorithms long before they become not even fully broken, but just weakened.
some cryptographers say ECC/ECDSA will be broken in the next several years......