Dropbox mentions their real competition in S1 filing: Microsoft, Google, Amazon, Apple, and Box. None of them seem to offer E2E. The competitors you cited aren't worth mentioning due to their small market share relative to the big players.

E2E might be feature customers start demanding in the future, but it sure doesn't seem like that now.

(though I wonder how many folks are using the big players on top of other encryption solutions like truecrypt/veracrypt/safe, etc)