I wouldn't say that this is any proof that GPL shouldn't be used. Yes, in long running bigger projects, who holds the code copyright can of course become an issue, but this also means that contributions to the code muse be respected. This is actually more acout copyright than license. There would be the same problems in a hypothetical multi-developer closed source code. It would just be harder to verify if the source and the contributions aren't available. It's just that closed source code is usually run by companies, which own the copyright of the code their coders produce.