If it has to be information stored in depth, then sure a KB/wiki is better.

IME, the majority of long emails should not be persisted in such places, because they contain a fair amount of errors. One could say that any long email thread should be eventually synthesized in the KB/wiki after the conversation has wound down.

And Slack/Teams? No way. If I need to write something long in response to a Slack IM, and email is verboten, then I'll insist on setting up a meeting.

I think we're on roughly the same page. Long emails are problematic. Realtime meetings or (possibly but not necessarily near-realtime) chats in a dedicated channel can and do solve email's problems.