Dropbox has a tough road ahead. With Google Drive, Microsoft Word cloud suite and Box, its definitely getting tough to survive by just doing B2C. With their recent acquisitions they'll probably pivot to DocuSign like model. But they are definitely losing their niche.
Enterprise is where SaaS companies go to die because they think that Microsofts of the world are just going to let them have the multi million dollar deals. That's a delusion.