As for their other paid service, Obsidian Publish, since all Obsidian notes are in plain markdown there are already many free alternatives.
So open sourcing would not harm any of those income streams. It's not about Obsidian losing profit. If you want to read the actual reasons they have decided not to open source Obsidian, they have talked about it on their forums[2]
[1] https://obsidian.md/help/sync-notes [2] https://forum.obsidian.md/t/open-sourcing-of-obsidian/1515/1...
Obsidian's income streams are based on Obsidian having easy-to-use easy-to-setup ways to sync and publish built-in. If Obsidian were open source, someone could fork it and remove or replace those built-in methods, which has the potential to harm their income streams. Whether it actually would and by how much depends on a lot of unknowns and is all just conjecture, but _if_ such a fork became somehow more popular than Obsidian proper, that'd definitely affect them.
Would it? Something like Zulip seems like a way better target in that case, but Zulip seems to manage just fine with open-source code and running their own platform people can pay for.
Not saying it is easy nor not hard, I'm just saying I don't agree with "either you do open source, or you go broke" because history shows us there are more choices than that.