You are missing one fact though. Out of 200 million users, at most 1-2% will be the paying customers (2-4M). The remaining 98% users has not yet found a compelling reason to pony up.
This set of users are definitely fickle. So the threat is more fundamental than you think.
It has the ubiquity (Windows, Mac and Linux Sync Clients) you are asking for and have better admin controls than Dropbox for Business.