Your logic seems to assume that there two disjoint groups of people:
1. Developers who work on the open source project
2. Developers who work on proprietary software based on the open source project
In reality these groups overlap. Developers contribute to the open source project, and they also work on their own software based on the open source project. If you look at the git logs of successful permissive open source projects, you'll see that a lot of the development time comes from developers who work for proprietary software companies.
That's true for some projects, but there are others who have a sole maintainer, receiving no other contributions or donations, who are expected to continue maintaining the work for free.
1. Developers who work on the open source project
2. Developers who work on proprietary software based on the open source project
In reality these groups overlap. Developers contribute to the open source project, and they also work on their own software based on the open source project. If you look at the git logs of successful permissive open source projects, you'll see that a lot of the development time comes from developers who work for proprietary software companies.