To me, Obsidian is a perfect example of how to get SaaS-revenue without making the user captive: All data is stored in an accessible form on the user hardware, and what you pay for is sync. X-user kepano (who is a/the creator of Obsidian) has some nice insights on this _file over app_ philosophy.
This is an effective, simple solution if the application doesn't have transactional constraints and authorization. The user is in full control of the data - and that's a good thing. It becomes a bit more complicated if the data has to be in a specified shape and can only partially be authored by any individual due to authorization requirements. Having a single source of truth on a server is quite a bit easier to reas…
It has neither of these things - it is aimed at individual (non-collaborative) use, and is intentionally very open and structureless. It is uniquely positioned to succeed with their chosen business model.