Say a project is started by one person, made open source, and becomes popular. They start accepting contributions from the public, including substantial features and bug fixes. They later move on and someone else becomes the lead maintainer. Who would receive the money in your scheme? The original author? The current maintainer? Divvied up among anyone who has ever touched the code? If the Digital Ocean tshirt giveaway is anything to go by, popular paid projects would be overwhelmed by "contributors" hoping to snag a slice for changing around a few words. It gets really complicated pretty quickly.
Say a project depends on one or more of these paid projects - does it now require payment of at least as much as the sum of its dependencies, and their dependencies, etc? That could add up quickly unless there's some scaling factor and you hope those projects make it up in volume. Surely there will be typosquatting projects which are (at best) wrappers for real ones and siphon off some of the fees.
And so on. All this is just to say that it's complicated and the details matter.