Yes, users would need to take some action-- otherwise how would you know if your coins were being moved to your new key vs my new key. :)

We know how to construct kinds of forward compatibility that could allow being spent with a new scheme but the specific scheme would have to be set in advance so they users could generate the future keys at the same time the generate the current ones. And, as mentioned, the existing choices aren't that great (for this application).

Is it reasonable to assume that many wallets won't get migrated and could eventually be seized by whoever (eventually) possesses a sufficiently strong QC? If Larry Fink is right, then that could be untold billions, assuming for sake of argument BTCUSD goes to $1,000,000 by the time such a QC is feasible.

If Satoshi Nakamoto did lose his keys to those treasure trove wallets, those will be up for grabs, as will anyone who lost keys.

Perhaps hoarded encrypted PCAP's aren't the only motivator for quantum opportunists.