I don't think apps have to be FOSS. It's super important for libraries, frameworks and other infrastrucuture related stuff to be FOSS, but you can always change a client.
I use sync, but I think they still should be FOSS, or at least open source. Just because when anything gets left behind or never fixed, you can take it and patch it up yourself. That includes clients too, lots of good clients that get left behind because Dev stopped and whatever they were interfacing with changed.
I don't think apps have to be FOSS. It's super important for libraries, frameworks and other infrastrucuture related stuff to be FOSS, but you can always change a client.
I use sync, but I think they still should be FOSS, or at least open source. Just because when anything gets left behind or never fixed, you can take it and patch it up yourself. That includes clients too, lots of good clients that get left behind because Dev stopped and whatever they were interfacing with changed.