At this point for me, it's lock-in. Even if Box, Google Drive, and OneDrive offered me all the main functionality I use with Dropbox, I still couldn't muster the time/energy it takes to investigate even though I'm a paying ($99 for 100GB or whatever) user. It's not just apathy, though, in the ~10 years I've used Dropbox, I have yet to ever have an issue, while at the same time, it's been such an integral part of my computer usage and workflow.