As for 40 years, it allows time for a given work to solidify in history. Multiple generations will have grown. If the work is relevant to multiple generations 40 years after creation, then it should belong to everyone. Twenty years is too short. It would be available to people who grew up with the first printing and to whom it is still relevant.

Basically copyright should be long enough for most things to become firmly irrelevant, outlive their usefulness, or made their changes to society before entering public domain.

A lot of people who oppose copyright would not like this because it intentionally keeps the stuff they want to use out of their reach until they would no longer want it.

For me, the purpose of public domain is allowing what has been important in our cultural history to be for everyone, not locked behind a rent-seeking corporation. But I also want authors to be able to keep control over their own works as long as it remains relevant to them.