Royalties are property. Like furniture and houses. It goes to whoever the will says it goes to. If no will, then to whoever the state decides should get it. Think of it like a trust fund or some such that pays out over time.
One problem with copyright extension law is people sign over their royalty rights for $X. When the copyright terms get extended, who gets the extra royalties? The purchaser or the original owner? What if the original owner died 25 years ago, how do you track down who owns the extended rights, etc.? Maybe no one thought to fight over nothing in probate all those years ago and now there’s money at stake. Etc.
(Of course smart buyers put in a clause that says they get the royalties even if the terms are extended. But the real world is not always that clean.)
What happens if you make a demo record that gets famous almost 40 years later? In this case, the singers were alive but what if they weren’t? So nothing in their wills about royalties. But still there should have been something in probate court, someone to track down, and if they died you track down their heirs, etc.