What do you define as a public good? All of these can be done with lower friction when copyright expires. Is that not a public good? Maybe it's a minor public good in your eyes that is outweighed by the benefits to the individual, but it certainly qualifies as something advantageous for the public.

> and it is abandoned, then who's to stop you building on it now?

A copyright troll who just sits on copyrights without doing anything about them. Again maybe you argue that it's their right to do so, which is fine and is argument that perhaps the hit to an individual good is not worth the public good provided, but it's clearly also a public good, however small, to remove that copyright troll.

(Also in the particular case of music there is a very finite cap on the number of copywritable musical phrases, since there are comparatively very few musical phrases that are compatible with Western musical theory and aren't just noise, we're already seeing lawsuits over this. With an infinite copyright system it's very possible that effectively all new music would have to pay royalties within a few decades)

EDIT: To put it another way, freeloading is basically just another way of saying "public good" (a library is allowing patrons to freeload off publishers, a public park is freeloading off property, etc.). It's just one that implicitly prioritizes individual good as well (which is totally fine, the two are often in tension and must be balanced against each other).

First time I've heard of copyright trolls, so it's worth unpacking that..

If you knowingly reuse a work, then I'd argue they are not a troll, they are a copyright holder, so go figure out a license fee.

What about unknowingly using something they claim? That's more subtle, and I buy the argument there. Copyright expiry would help to avoid nonsense claims. But maybe sane courts can adjudicate that - then again Sane Courts (laughing).

But aiming overall copyright expiry to limit nonsense claims, would also prejudice real works that need real protection. So do we lose more than we gain?

So assuming expiring is good - lifetime of author plus a bit?

> You want to write a song, go write on, you don't need to mix someone else's.

Why I don't generally like bringing up fiction as proof of my words, Melancholy Elephants story has a very clear and explicit answer why this is a bad idea: http://www.spiderrobinson.com/melancholyelephants.html

On top of the examples already in the story: Disney's empire is built directly on the back of directly using public domain works or remixing existing works. And yet, you want to prevent anyone from doing just that.