Wait, why are you mixing the two? You can have the software be under an open source license, yet still not be a monk that has taken a vow of poverty, it's not black and white.
AFAIK (as a long-term Obsidian daily user) Obsidian makes their money on various things attached to the editor/viewer itself, but don't actually charge for the editor/viewer. Even if they did, they could still slap a FOSS license on it, and continue charging for the parts they charge for today.
I'm guessing it's something else they're worried about though, rather than those things.
I agree with your very last part though, but I don't agree you cannot make it open source at the same time.
Reading their other comments, they are under the mistaken impression that every line of code written by a human should have a dollar sign attached to it.
No consideration given that lots of people contribute voluntarily to open-source projects or even release their projects/code for free because they enjoy writing code and engaging with the broader open source or free software community.
"Wait, why are you mixing the two? You can have the software be under an open source license, yet still not be a monk that has taken a vow of poverty, it's not black and white."
I don't think they are mixing the two. 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.
It's very hard to use this model to actually build a profitable company.
The only open source projects that can actually sustain themselves financially get handouts from large corporations (or are eventually purchased by them).
I'm mixing the two because I think developers should value their time and profit from the value they add. I want them to build viable businesses so they get wealthy from their efforts and can continue keeping useful products alive.
There's no value to their business to open sourcing the product. Open source risks losing customers to knock-off competitors or fragmenting their plugin ecoystem (which is a lot of Obsidians moat).