Sorry, it’s not clear to me what the rationale could be for comparing Zettlr as a commercial entity. Zettlr is not trying to be that.
I would argue that it’s akin to discussing how to make money from running a hotel and claiming that a community homeless shelter proves you can’t make money by offering people a place to sleep. Yes, people can spend the night in both however the scenario and goals of each establishment are completely different. Perhaps there are things they can learn from one another but there’s no sense comparing them as commercial business models since only one of the two fit that description.
An interesting comparison that might be apt however, would probably be something like Standard Notes (https://standardnotes.org/). I don’t know a great deal about them (so maybe the comparison would break down at some point) but they’ve certainly been around for while, are focused on longevity, and provide a free and open source note / kb application while operating within a commercial business context (albeit the StandardNotes model provides different services than Obsidian’s).
There are business models that rely and operate on open source but the concept or practice of open source itself, is not a business model.