The mental model of slow decay into copyable is complicated. The code base is already a wash with contributions that have low copyrightability in US case law anyway, things derived directly from external architecture or documentation in barely novel ways, common system interfaces, and so on. LLM output is another output of this kind of class and sure sometimes large contributions happen that seem more significant, but they likely have little bearing on the work as a whole given it's earlier design provenance (at least for established and massive projects like this).

What is likely to get more muddy over time is the accuracy of any copyright registration, and the enforcement of copyright infringements on portions of the whole. These are already complicated cases and definitely so for compilers with so much "scènes à faire".

It's not clear how much this has a negative impact on cases around the whole, which tend to be the more important cases for the four freedoms that, while they have other intentions, have a primary intention of ensuring that the whole continues to be available for redistribution and extension in perpetuity.

I do not think that there is a clear link between these two areas at all, and the GPL's most important intents may be far safer long term than concerns of dilution suggest.