Maybe there is business for developing the missing features. I would pay for that. But there is also a requirement for certifying that the data of the software

Maybe there is business for developing the missing features. I would pay for that. But there is also a requirement for certifying that the data of the software can't be altered, and you are not allowed to certify yourself. This is the bad catch 21 situation. Even if it was possible to ensure that data can't be modified on your computer, something like android protected storage, I guess most people would not like that because if you use open source it is to have control over your machine. So the only solution is the "cloud"/hosted solution by a provider, but again, if you use open source in the first place, probably it is because you don't want some external people to have control over your data.