All bitcoins that are not moved for a certain period of time (20, 50, or 100 years) should be presumed lost and then recycled through the usual mining process. I'm not sure if that can be achieved through a soft fork.
That's dumb. Private keys are forever.
Value comes from scarcity, not abundance.
To quote someone who might know a thing or 2 about Bitcoin:
Lost coins only make everyone else's coins worth slightly more. Think of it as a donation to everyone.