I think the key point that you are making is that projects which need more help should place higher priorities on working through PR and issue backlogs as those

I think the key point that you are making is that projects which need more help should place higher priorities on working through PR and issue backlogs as those are a primary way to taking people who are willing to help and converting them into more dedicated and integrated maintainers. Letting those backlogs languish because you don't have enough volunteers becomes a self a reinforcing cycle that becomes harder and harder to break out of.