> This can make it harder to recruit further contributors as there is a two-clays system of contributors.

We'll see. I haven't seen any reduction in contributions (not that the project gets lots of contributions because we're the same as every other Open Source project, most consumers just consume). Also, note that the fee is just for maintenance. I've seen near 0% contribution rate for all Open Source projects to "maintenance chores". Those just don't fall into the "scratch your own itch" class of problems.

> Accepting money make sit a business transaction, if I accept somebody's money they have demands towards me. Then I got to work on it.

True, but I've been committed to my project for over 25 years and I want to continue to improve the project. The fee has really helped keep that motivation up (aka: sustainable). The reaction has been mostly positive which is also a plus. :)

> But of course the volunteer free model has sustainability issues ...

Agreed. I think the OSMF is a good way to tackle exactly that issue.