Merely attempting to enforce a copyright that you legitimately hold against one instance that might be fair use, and then having a court find in a real case that it was fair use, would be enough to lose copyright of the work entirely in your view?

That's so disproportionate that you might as well just abolish copyright.

Also, most of the world doesn't have fair use like the US does, though many places have similar but weaker provisions in some respects. US fair use law is somewhat controversial, because it arguably doesn't comply with international obligations.

> That's so disproportionate that you might as well just abolish copyright.

Ideally that's what we would do.

Maybe one day we will, but first someone had better come up with another economic model that demonstrably supports creation and distribution of new works anything like as effectively as copyright has. There are a few ideas with potential, mostly in specific niches, but no-one has come anywhere close to reaching that goal in the general case yet.

No, that's not what's being argued will happen.

I own the copyright on the song X.

Person A uses the song X in a fair use way.

Person B claims A is infringing.

Person A challenges and wins.

Then I lose my copyright, without any involvement?

Why would you lose your copyright in that case? Person B loses their (non-existent) copyright to the song. This would only fail if it'd be literally "all copyright on song X gets wiped, without looking at who owns it", which as you pointed out is obviously a non-workable interpretation of the proposal.

You are replying to the person who proposed the “lose copyright” idea.

That's exactly right. The person repeated the concept, in a comment dated later than the other replies to the original; I tried to clarify why it wouldn't work.