Where did I imply that? There are tons of options for business chat. My point was only that email isn't even in the running.

I've never seen any email system used securely. Even smart people will send sensitive info to a personal address instead of corporate, for example. Email wasn't purpose-built for controlled internal comms.

Part of this is a UI problem, but it's still a problem.

I didn't say Slack is the answer, either. Just that email isn't a serious competitor to Slack.

And how does that work for your average person using outlook / google for email?

If that were viable people would already be doing it. It's not, and so very few people are. I wish this weren't the case but it is.

Companies are generally at more risk from failure to produce message content in a lawsuit, regulatory action, HR investigation, etc. than from eavesdropping. "Secure" in a corporate setting means subject to IT's surveillance, filtering, and archiving. If anything, security's job is to make sure employees aren't using things like PGP, personal webmail, Facebook, etc. to evade corporate retention policies.

Maybe mysterious quasi-state actors will steal your secrets and outcompete you decades from now, but they could probably do that anyway. The SEC will certainly fuck you up right now if you can't demonstrate adequate controls on regulated employees' communications.