Well they'd just release it under a non-commercial license. The majority of their income comes from Obsidian Sync, and someone can't just host their own version of Obsidian Sync for all the Obsidian users for free. And there are already self-hosted alternatives to Obsidian Sync, in fact Obsidian even endorses them themselves[1].

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...