You raise a valid point. I think what it ultimately boils down to is to give your users the feeling that even if your service stops (and there are many reasons why it could), all the hours they put into your product are not lost. The criteria I listed are
factors after all, and how I factor them in depends on the available alternatives and the work I will put into it myself and whether a online-solution actually makes sense there.
A good example are note taking apps. My notes should be private and I want to be able to read them ten years later. For me your product would need to add something valuable to this that a filesystem and a bunch of files that I synchronize myself can't do. As for now there is no note-taking app I found where the benifits outweight the preceived loss of privacy and reliability. The online thing can make sense, but syncing my phone with Nextcloud works even better, so I don't really see why I'd need it.
This is potentially different with an app like yours, because the benefits of using your app vs using e.g. a spreadsheet seem truly tangible. Using JSON and allowing export at any time is a huge plus. Having it a web-only kinda makes sense, as your app seems to be geared toward teams (and any serious parts managment makes a lot more sense when you are not alone).
While the additional work that would have to go into documentation and programming if you were to offer this as a self-hosted variant is non-trivial, from my standpoint offering the option to self-host can also be seen as an act of communicating: "Don't worry, whatever happens to this project, you will not lose the time invested".
I am not obsessed with this kind of reliability, but I just want to avoid the future hassle of having to deal with this, especially when I put a lot of time into it.