Cheaper and more robust isn't really what Dropbox has been competing on.Great UX, feature set, support, and general marketing is what will make the difference.
Of these I think UX is the most significant. UX is where the proprietary walled gardens and centralized services always win.
Good UX is unbelievably, grindingly hard. If you want to make a product with a good user interface and overall experience, 90% of the work is going to take you about 25% of the time and the remaining 10% is going to take you 75% of the time. Most hackers decide a product is done when it works, but in reality they're not even half way there.
Apple is probably the overall king of UX. If you look at their process and their culture, they obsess and obsess over tiny little details that are irrelevant to core functionality as understood from a purely technical point of view.
Closed silos have a clear revenue model. That enables them to invest in pushing product quality and user experience the rest of the way, past where hackers and enthusiasts would take it and into the realm of polished usability that real customer bases demand.
Who's going to pay entire teams of designers to obsess over the filecoin.io client's "experience"?
Mastercoin pays its developers using currency created in the first block, in a system of bounties managed by the Mastercoin foundation. You can contribute a patch and get paid. Go read on their website, it's interesting.
But it's absolutely not more robust! What if all the 'servers' having your files go into limbo at once? Dropbox will make a big splash and you know who to go after, but with filecoin you're on your own. :-)
