Their defensible advantage is two-fold:

1) they developed a technology that in theory should be easy to duplicate, but in practice is extremely hard. Large teams always mess this up (even world-class teams, e.g. at Microsoft, which has had several stabs at this), and small teams often don't have the skills to work with so many orthogonal elements at the high competence level required.

2) They have supportive users signed up on yearly contracts. That makes them very resistant to competition.

A Dropbox competitor would ahve to be 10x better for people to switch. That's hard to achieve.