Something that makes forums a bit different is that it costs the owners when people use the website. Unlike Blender, Firefox, Linux, etc… A server host can’t just make the forum available, then set and forget it, they either have to pay a huge fee to some host like AWS, or have a huge stockpile of computers in their basement.

Back in 2008 I met a bunch of the VLC devs at a KDE related open source software conference. They talked about their experiences getting approached by companies with "fuck you" levels of money with offers they couldn't refuse -- and yet refused. In 2008 it was about bundling spyware with installers, largely. I always admired their stalwart refusal to bend.

Side note: this was shortly after they'd completed their transition to Qt as their toolkit. They stole their little volume control widget from KDE's media player, Amarok. The beauty of open-source and cross pollenation. I expect Lemmy and kbin and others in the fediverse will freely cross pollenate too. In the end, open source wins.

The difference is you cited software projects, not hosted infrastructure. A person can contribute to a FOSS dev project and not incur expenses dependent on end-users activity. Hosting a fediverse application isn't like that, somebody has to pay for the hosting and the hosting expenses will scale with user activity.