One caveat though. Out of 200m Dropbox users only 1-2% of users will be paid users (2-4m). The remaining 98% of users have not found a compelling reason to pony up yet. These users are fickle and have less inertia than paid users. So the threat is real as the article suggests.
I assume you're being sarcastic. Non-paying customers do server at least one purpose for Dropbox, word of mouth advertising and the fact they tell others "I'll just share the file you need on dropbox. Go sign up, it's free". All these non-payers, and the people they refer, may start paying at anytime in the future for whatever reason.
All users of Google search are non-paying, yet competitors have found them remarkably difficult to dislodge.
And in the other direction, Gmail is a big popular email service now, but it took many years, and a dramatic value proposition (1GB for free!) for them to make inroads against Hotmail and Yahoo.
I see no reason to believe that free users would be any less likely to be loyal than paying users.
I have something like 55GB of free space on Dropbox. I would love to pay for Dropbox but I simply don't use even 50% of that free space so I have no real need to upgrade.
