That's entirely different Tot, this isn't about pay to play, this is about a monthly income for the devs to facilitate a better game. The donation system works in an entirely different way, and doesn't encourage recurring payment, if someone donates they'll most likely donate once, because they know that even if they donate more than once, most people will not, and as such nothing real will change.
I just want to emphasize, that this is has no correlation with a donation system at all. Donations are a way to thank someone for doing something. Monthly fee is a contract for a service; improving the mod. If you compare those two things then you've misunderstood my point.