Maintainership of a free software project can be very taxing so it’s refreshing to see attempts to address that that aren’t intrinsically at odds with the free software movement. Remember that users of free software have no entitlement to anything other than source code. There is no requirement in any free software license that a project have maintainers, take bug reports, accept pull requests, offer support, etc.

This proposal could totally backfire though. There will be users paying 5 Euro per month and then demand on the issue tracker that major changes get implemented overnight. Or people who contribute with good bug reports that are unable to pay money, so problems remain unfixed. There might be a way to balance things so it works out, but that will take time. In any case its worth experimenting with different approaches to get open source betterfunded.