The problem with dual-licensing is that it impedes collaboration. If one contributes a big feature, shouldn't they also get some cash from those sales? If so, who decides how much?

That would be a negotiation between the library maintainer, who is running a business selling commercial licenses, and the potential contributor, who wants to be a paid supplier to said business. If the negotiations fail, the contributor’s code is neither merged into upstream nor sold as part of the commercial offering.