"users won't adopt it if they have to pay for it"
There is truth to your comment: there are plenty of users who associate "open source software" with "free software", and that 'free' is free as in beer.
Note how they all manage to receive donations in order to carry on their work? Hosting software isn't free, hosting a website not so much either and here we are: I can visit those websites and my ad blocker doesn't have to block one single ad.
As you can see the FreeBSD foundation even managed to raise $270,000 worth of donations (at the time of writing). That's all coming from people and companies who care for the project, who realize that they can be saving money by using it and then spending some of that money on the project itself.
Please note: the only reason I focus on the FreeBSD foundation is because this is my personal favorite, but my story holds true for all the other foundations as well (main difference is that I have no idea about their current funds).