How about this for an idea to partially address the issue with dormant bitcoins:

Allow users to voluntarily opt-in to a wallet setting that causes them expire if there is no activity in xyz years.
Create a similar setting that causes the wallet and its balance to gradually expire starting in xyz years.

Force wallets with un-usably low balances that have been dormant for say 40+yrs to expire.
Force wallets that have been dormant past the normal human life expectancy, let's say 100yrs, to gradually expire by say 5% every 10yrs.

I expect the last two suggestions to be rather controversial, but the first two shouldn't be since they would be voluntary measures.