I don't understand your target audience: > In our age when cloud services can shut down, get bought, or change privacy policy any day, the last thing you want

I don't understand your target audience: > In our age when cloud services can shut down, get bought, or change privacy policy any day, the last thing you want is proprietary formats and data lock-in. Concerned about proprietary lock-in when it comes to file format, but not when it comes to software. Seems like you could make some technical gains without losing audience by getting consistent. You could ditch the "open standards based file storage that you manage yourself" idea and gain flexibility to do your own thing there. I bet people would expect you to handle sync in that case, but that's not so bad. Alternatively you could go open source for functional parts of the app and build a development community to support you and your users.