By the time quantum computers can get to the point of cracking bitcoins encryption - it would have likely developed quantum resistant cryptography - as far as I can tell. I think bitcoin is already starting the migration - but could be wrong.
Thought that could be the case, think early unused could be very vulnerable. Perhaps the ones in the tip could finally yield some value or even satoshi cashes some in?
I think this is likely the biggest risk. It is probable that for quantum resistant encryption to be effective coins will need to be moved to new "resistant" addresses. So old untouched coins will be vulnerable.
By the time quantum computers can get to the point of cracking bitcoins encryption - it would have likely developed quantum resistant cryptography - as far as I can tell. I think bitcoin is already starting the migration - but could be wrong.