You can make it FOSS and still have some sort of subscription for syncing between devices for example. Tasks.org did it like that.Not to mention that you can run something on donations like lichess or F-Droid and have some extra money.
I don't wanna take part into all that discussion I'm more interested in why there's so much hype around this app. It's beautiful and stuff but not open-source and I bet there's no gamechanging features.
They probably can but only after manual analysis.