Open source projects are under financed and their maintainers are overwhelmed. I would rather read proposed solutions about that because that seems like the more important problem.
Freemium format. It floats the WordPress plugin ecosystem. That ecosystem has $1 bn size. Likely larger now since that statistic was made 1-2 years ago.
You give the base software and most if it free. You provide paid addons for niche, corporate, business use cases etc. Everyone is happy - the userbase gets their software free, advanced & business users have advanced features that could not have made it into the main software in addition to having paid support. And most important for everyone - the project & its organization can sustain itself and it can keep maintaining its software.