So limit the changes to works the copyright owner intentionally distributed to more than $threshold people. Have it only apply to situations where the number is tracked already, e.g. due to commercial obligations. Or something. This does not seem like an insurmountable problem.
Then you're bringing into the question why the rights that someone gets is different depending on how people exercise them. What's the right threshold? What does it mean to intentionally distribute? Are we going to have protracted court battles over what tracking numbers are accurate or valid? Could those who want to abuse this (big business) simply work around it by making it infeasible to directly measure?
And also, the copyleft complication too: If Bob writes a FOSS app with a GPL license, and it becomes popular, but then Bob retires and stops distributing it, does the copyright expire?
I don't think the problem being discussed here is insurmountable to solve, I just think the proposed solution opens a whole new can of worms. I think a good start at a solution is to first shorten the duration of copyright.
