The only way that Dropbox can stay independent is that they develop some email solution. Small companies (which I believe are core of Dropbox business) need just 3 things: email, docs and spreadsheets.
Dropbox had acquired the Mailbox email client in 2013, but then shut it down in 2015. This was a very annoying turn of events for people who used the app prior to the acquisition.
Funny enough I think Basecamp must have realized this exact niche a few years ago because they now offer all three of those for small business.