The drive-mirror-backup split is the right frame and I would push it one step further: the reason it collapses into a suite is conflict resolution.
Two devices edit the same file offline. Something has to decide. The plain file server does not have this problem because there is one copy and whoever saves last wins, visibly. The moment you add offline access on a phone you have created the problem, and solving it properly needs versioning, which needs storage policy, which needs a UI, which needs accounts. That chain is why the simple thing becomes the big thing.
Every project in this space that stayed small stayed small by refusing offline mobile editing. Look at any of them and check whether the mobile app is really a viewer with an upload button. It usually is, and that is not laziness, that is the only way to stay out of the chain.