there seem to be two schools of thought that I have seen over the years.
If you build it they will come.
As I see it the first option is more of a FOSS from the start, scratching an itch that turns into something with a large community that has the potential to have some income (help/support) or funding/donation support (LibreOffice/MariaDB are two bigger ones)
Can we make money off of this thing.
This could easily be summed up in IBM eats Red Hat but that may be a cheap shot at IBM. I would also say that it could be the easier and yet harder question to answer by looking at Twitter/Diaspora/Fedverse. While one of the three are not open D* did get a few rounds of VC funding after they had MVC in the public and Fedverse had donation support at some level for the various flavors of clients but I don't think anyone is getting rich yet off of any of them, even Twitter.
The paid support model or the SAAS model with FOSS seem to me to be the most likely model that one could either start something or open something and be able to make a living off of it at some point.