The fossify apps are legit, I use them daily.
This is the same or similar situation as OpenOffice → LibreOffice or OwnCloud → NextCloud (and to a lesser extent but more similar scenario, Audacity → Tenacity or various other forks).
This is what true open source looks like in action.
I don’t think it is. I know a few people like this, and im heading in that direction myself. The only kinds of “ads” that work on me are when a number of equally nerdy people I know find a new thing, and they’ve demonstrated that it has helped them with something or they are genuinely enjoying using it. Like 3D printing. Its semi-pointless most of the time but it is a genuinely fun hobby, which when combined with 3D modeling and post-processing skills becomes an actual craft. I didn’t get into it until a good number of people around me did.