(But more seriously, even with a good MUA and either client-side or server-side search, the way mail messages are constructed, with tons of reply quoting, makes searching them a pain.)

In lots of business cultures there's no expectation to search email, because it's so hard to get useful results that way, so if you send it in an email you can expect to have people just ask you to send it again. Even if you can use search, it still sucks.

Slack is not perfect but it's at least designed for search.