This is not an all or nothing issue. For example, if you store sensitive videos, but rely on Dropbox's ability to transcode and preview the video in a web browser, then something like Cryptomater would not make sense. Encrypting on wire/at rest is the best you can do unless you run the transcoding yourself.

Dropbox could support both modes, but the company has obviously made a decision to prioritize one over the other (at least for now).