Everything is CC-BY by default, copyright is opt-in.
If someone opts in to copyright, it ends whenever they die (I'm going to be nice and say "until the last person dies" for a group project).
I hate copyright, but understand that some people really want to keep their work for themselves. Maybe they can do that - in a world where copyrighting isn't default, we'd have so much to choose from that we wouldn't need the content made by the kind of people who decide to prevent sharing their work.
I think your first idea is very interesting. It essentially turns public domain (with attribution) into the default. I wonder whether you have to opt-in to copyright before sharing, or if you get a small indow to opt-in after sharing. For example, a creator shares something online that "goes viral" unexpectedly without specifying the work is copyrighted. Do they get a period (say 60 days) to specify "wait, this is under copyright"?