I think the key thing you are missing here is that most successful open source maintainers don’t lack expertise, they lack time. They are generally capable of improving their projects in any direction they want without help. Mentoring new contributors is basically always a loss in the short term as far as the maintainer is concerned. The project does get improved, but it generally takes longer than if they did it by themselves from scratch, and the improvements might be in an area they don’t much care about. The payoff for the maintainers only ever happened in the long term if the mentorship succeeded and they get a viable independent long term contributor.