Not all must be online, just 2. Also, Dropbox isn't open source, nor Box, Google, Microsoft. I don't think that's really a requirement for success in this space.

Only one computer needs to be online for the sync to work. This is a good thing because your files never go to a third party. You don't have to trust Dropbox or anyone else with your data. Plop it on a couple of servers and you're good to go.