There are ways to introduce a paid layer of services above and beyond the basic product without needing to lock away features of the software itself as you said

There are ways to introduce a paid layer of services above and beyond the basic product without needing to lock away features of the software itself as you said, though. Things like cloud-based services that cost you money to offer. I'm working on this now for one of my projects that has quite a bit of debt in my name for the first several years of hosting when it was more expensive for hosting and bandwidth. While donations may be preferable, one unfortunate thing is that while donations and advertising on your hosted site can add up and can pay for hosting and a few niceties in some cases, they often don't bring in enough to support even a single developer. Or pay off back debt in my case. Many open source projects wind up abandoned due to the developer(s)' inability to make ends meet from the work they are doing. Real life responsibilities... job, college debt, family, etc... tend to pull in the other direction. Many open source and freeware apps in the Windows world are turning to less-preferable means of supporting themselves such as bundleware. In the mobile world, they're trying their hand with more intrusive in-app advertising and in-app purchases.