> This is a bigger problem, but in practice I assume it wouldn't happen because if someone were to actually put in weeks or months of work into significantly improving the project, why wouldn't you just hire them or pay them ? After all, the whole assumption here is that this is a model for a for-profit endevor.
This is getting a bit hypothetical, but here's a question. Lets say you're an experienced software developer and you are using an application called "A Large Well-Known Word Processor" that is dual-licensed (A)GPL and proprietary, as explained in the post.
You might really want to add a feature that would save you time for a personal project, or just for your own sense of enjoyment.
Perhaps that feature had been discussed in support forums / issue trackers, and you want to add it as a kind of pro-bono / personal achievement in spare time.
And perhaps the company wouldn't want to pay for it to be developed themselves, perhaps because it competes with a revenue-generating plugin in their ecosystem developed by a partner company.
As I say, it's a little bit "out there" in terms of use cases, but in terms of the freedom to modify, improve and have input into the software that people collectively use, it's a valid one.
Another commentor elsewhere on this thread mentions a potentially-related problem regarding jurisdictions where it's not legally possible to remove your own copyright over materials you've produced.
That actually initially strikes me as a sensible policy, since it reduces the possibility of people being pressured (knowingly or unknowingly, with or without compensation) into losing control of their work.