Aside from sending links to download files, which isn't a deal breaker, selective Sync is probably the only reason why I still have Dropbox, along side iCloud Drive.
Right now (as of macOS Sierra) iCloud Drive backs up 99% of the files on my Mac..all built in Applications have their own iCloud Drive folder, which is great, and now my Desktop and Documents too, so I can be as disorganized as I want and still be backed up (Great!).
The problem? I have several other computers, mostly small storage PCs, that I want to sync files to. But I don't want my entire iCloud Drive downloaded to those computers. So I still have a Dropbox folder or two that are synced to those machines.
This is a common use case and I imagine it is one of the main reasons any iCloud Drive user still has a Dropbox account. Oh, I also like the feature that collects ScreenShots into their own folder.