> If they open sourced it, there would be immediate competition. Anyone could fork it and circumvent/compete with any premium features they might want to add to it in the future.

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.