Just like every other existential threat to bitcoin QC will threaten other more important things, like infrastructure, first.
It's a non-threat.
And then the community could decide “if you didn’t update your addresses before quantum computer attacks were viable, even though you had 5 years to do it with strong evidence that it would be necessary eventually, and lost your coins as a result, then sucks to be you”
It seems like the only options is to either just let it happen or make those bitcoins disappear.