Look up “unclaimed property”, there is a whole complicated bureaucracy dedicated to tracking this stuff and letting heirs claim it. Each state of the US has a separate web site where you can search for anything left in a dead relative’s accounts.

Claiming it is easy for small values. It gets more complicated as the value rises, I have enough left in this situation from a grandparent that I am putting off getting something called a “judgement of possession” to acquire some funds.

This bureaucracy does not, of course, exist for cryptobucks; creating one probably will be mandatory if it is to become an actual part of the financial system instead of a cascading pile of scams, and it will involve a whole lot of things that are anathema to the anti-government attitudes a lot of crypto true believers have.

The short answer is inheritance happens. If there's no co-signers, named beneficiaries, will or heirs, the state will eventually appoint an executor who will settle any outstanding debts of the estate against the account, and then I think the state itself is the ultimate inheritor.

Most states have a notion of "unclaimed funds", and you can check to see if you have any money sitting in the state coffers from a dead relative or a settlement.