Dropbox doesn’t encrypt your data. There area bunch of other companies that do encrypted cloud storage, eg keybase
If your threat model mandates that you use end to end encryption, go for it, and choose something other than Dropbox. But saying the encryption is pointless just because the organization that manages the keys could become “evil” is hyperbole.
Edit: to be clear, I believe it is an option on enterprise accounts to use your own key.