I think many open source projects already experience two buckets of contributors which maps nicely to the two class distinction inherent in this model:

1) a bunch of people who contributed one or two PRs, but it took the maintainers more time to review/merge the PR than the dev time contributed

2) a much smaller set of people who come back and do more and more PRs, eventually contributing more time than it takes to review their work

A major existing reason to review PRs from class 1 "once or twice" contributors (perhaps the main reason?) is that all class 2 "maintainer-level" contributors start as class 1.

I agree there's an awkward middle ground here, now you have to define where the boundary is between class 1 and class 2, but I think if you were able to graph contribution level you'd find there's already something of a bimodal distribution naturally in many projects anyway.