stated above, if the current cryptography associated with our privatekey(s) are known to
be very vulnerable to brute force and theft by an advanced QC in 5-25 years?
Can't it be assumed that at some point in the future we will all need to switch to stronger
privatekey cryptography? Are you and others assuming that our current encryption type
will be equally as strong from generation to generation, passing those HODLed privatekey(s)?
That's a fair point. I'm a little bit skeptical about QC and whether a future, actual QC can indeed identify private keys efficiently. But I could be wrong. If I'm wrong then you're right, we'd need to conduct a wholesale migration of bitcoin funds to new QC-safe algorithms.
So in that case I agree we need to migrate and anyone who doesn't would likely lose their bitcoin. But I would urge that such a migration observe certain principles - that a maximum amount of warning time be employed, with widespread advertising within the bitcoin ecosystem and clear instructions that are as simple as possible on what needs to be done. And if possible, I'd support having a process by which someone who manages to miss all the advertising (I can think of many valid reasons why this might happen) can still migrate their bitcoin or otherwise be recompensed after appropriate review.
(The transition from old Piggycoin to New Piggycoin in the altcoin world is a case in point. As I recall, millions of new coins were unclaimed after 12 months, despite it being a young currency that you'd expect holders to pay close attention to. As cryptocurrency matures this problem could grow more severe.)