It doesn't matter if the technology exists (which it does) what matters is if the vendors will cooperate (which they won't).
A universal protocol would only handle the "data in flight". The other issue is "data at rest" which is storage problem.
As analogy, SMTP is a universal protocol and yet email giants like GMail/Hotmail/Yahoo exist. They store emails. Average users don't want to point DNS MX records to their laptops running home mail servers 24/7.
- Dropbox-the-client-software simplifies "data in flight".
- Dropbox-the-cloud-storage simplifies "data at rest".
Since storage costs money (e.g. see Amazon Cloud Drive drops "unlimited" plan), it means there would be conflicting business interests from OS vendors at that infrastructure layer. This why a company that was _not_ Apple/Microsoft /Google/Redhat such as Dropbox was able to fill a gap.