Not at all absurd.

Maybe not through crypto but otherwise perfectly achievable. No reason to achieve this within a file. If you really want to, some fancy solution involving blockchains and smart contracts is probably possible but there is no need.

Require by law all who desire copyright protection to register the work with a governmental agency and place a copy of the work in escrow at an archive as a condition. The archive knows when the copyright expires and starts serving the work to the public from that point forward. This is what to do if a state cares about public domain.

The status quo of corporations abandoning works to bit rot, actual rot, misplacement or fires for decades is strictly worse.