Today I had to search for Slack messages that were sent long before I was hired to solve a problem at work. In an end-to-end model, you're only going to have logs as far back as you were there to receive.
Allowing different security models for different groups would make sense, as long as you could communicate the security models to users and admins somehow.
e2e for direct user to user makes sense.
I disagree, corporate chats still have plenty of reason to want access to historical user-to-user chats. Ignoring the cynical reasons, institutional memory isn't confined to channel chats.