Seeing that Dropbox is one of only two YC companies to ever go public, they have never been profitable, and have said in their own filings they don't know when or if they will ever be profitable, the pessimism seems warranted.

Not to mention, it seems like Steve Jobs was right about DropBox. It's a feature not a product. For the same price you pay for DropBox, you can get the full Microsoft Office Suite plus 6TB of storage. The same with Google Drive and GSuite.