It depends on what license the project is using. Some licenses are very permissive, meaning there’s lots of ways they can be abused. For example with MIT/BSD licenses there’s no provision to share the code with the final product so they could drag their feet releasing parts of the code or hide them altogether. They could also resort to tivoization, NDAs, commercial plugins and all kinds of shenanigans.
Look for example to the Plex and Emby projects which were originally open and went commercial later. The way they did it is why there’s a lot of bad blood in the community to this day.
I’ve also personally been involved with other projects where someone tried to take them commercial in a less than graceful way, shall we say. It’s never pretty.