Thanks for your note.
You may have missed my point #2 :-) Note that most Dropbox users have the free tier. That’s a loss leader that is designed to drive you to upgrade, and Dropbox aggressively pushes you to upgrade, even to people like me who are already paying for the 3TB plan!
For now, few have heard of Maestral. Combining that with the friction of downloading and using a 3rd party client makes them a small threat. But Twitterrific became wildly popular, and that was that.