Being accused of plagiarism seems orthogonal. Even if I assert IP ownership of my code, someone else can falsely claim it as theirs. They are wrong in both cases and in both cases it'd have to be settled in court.

If nobody owns the code and everyone has the same rights to it (per the parent) the person copying the code would have the same right to claim they wrote it. And there are definitely people who will do this when there is money and reputation to be gained. It happens all the time even with the IP protections we have.

My point is that the blast radius of losing those protections goes beyond open source.