People would be stupid not to move if the encryption was under threat and we could actually force them by requiring them to change addresses if all wallet software developers accepted this change. It would benefit the Bitcoin community and would be the only time I would suggest a forced move to prevent further loss of coins but if people don't move and lose their coins then that would be entirely their fault for refusing something which is so blatantly obvious.
There are without question millions of inaccessible coins. I'm sure the owners would absolutely love to move them but they don't have the ability to any more. The only people who might be able to in future will be those pesky quantum people.
It's a very pertinent point that I hadn't thought through properly. There's a lot of low hanging fruit that's effectively helpless and dead and then instantly wakes up available to anyone with the right gear in this scenario. People might expect white hats to claim them first but even then that's still not a good look for a supposedly impregnable and immutable system.