Dropbox's weakening position isn't necessarily an indictment of the idea, just that they're in a tough market when you consider both Google and Apple offer (comparatively cheap) cloud storage that is well-integrated into their smartphone platforms. Also consider that the success of Google's and Apple's offerings has proved out the value of the idea many times over.

Also, Dropbox was an early mover. That worked well for them in their early days, but the consumer space is saturated by better options. Dropbox (and Box) are often better suited to enterprise use, where some of the more advanced features (that regular consumers mostly don't care about) are important.