I'd argue Dropbox became as big as they are thanks to mobile taking off.

In a computer only world there are myriads of other solutions, elegant or not.

Most work computers were permanently plugged into network shared folders, and would have over the VPN access for on the road salesmen etc.

Home users mostly didn't care about cloud storage or shareable folders, those who did could get away with ftp (basically supported everywhere, like straight in explorer windows)

Dropbox flourished because most people got a second device, always connected, but with no decent file management. Many of us used Dropbox not even for sync but just to properly handle files.