Why not just 30 years? Patents get a flat 20.

Not that I'm arguing for 30 per se, just that I don't see what goals of copyright would be advanced more by adding an "or until death" complication.

Well imagine you write a book in your 20s or 30s and it only becomes popular after a couple of decades. The publisher gets to pocket all the money.

Somebody decides to make a movie based on your book? You get nothing at all from it... The movie bit would be problematic even for books that were reasonably popular at the time. e.g the Witcher adaption came out almost exactly 20 years after the last book, for GOT it wasn't that far from being the case as well (at least for the initial volumes). Studios would be incentivized just to wait a couple of years to avoid paying anything.

I think it could be reasonably to have a fixed limit if the rights are held by corporations, though.