A simple solution is to close all feature requests with a tag that lists is as a request, and a comment that explains that you may go through the requests some day when you don't have many bugs to fix. Or if a customer is willing to pay for an extra feature. Maybe even add a tag to indicate that this feature would be a good place for new contributors to start. Of course, you then get the same feature requests multiple times.
Note however that the scenario in the OP that leads to ruin starts with an external contributor contributing a feature. Someone other than the maintainer contributing the PR doesn't really solve the problem; the features still leads to complexity and maintanance burden that inevitably falls on the maintainers.
While I understand and agree to the solution you provide, I would dare to call it a workaround.
Since Github default culture is to use issues in a certain way, in which the maintainers are actually the bottleneck, most people will keep using it thinking it's the standard and correct way to do it.
Unrelated to the above, you mentioned the paid extra feature, I really wish to see this implemented in a source forge to see how it goes.
I've seen some projects that say almost exactly this: Feature requests will be closed. If you have a contribution or patch, I'll look at it. If you want to pay for something, contact me.
