Except you can only find stuff by spelunking with search, hoping you hit on the right set of keywords and then scrolling through a jumbled mess of hits. Wikis are organized and have navigation. Also being at a company that uses email and Slack, I will say it is very very rare that anyone actually uses Slack to search for conversations they weren't in. Email has other weaknesses of course but in general I agree with the point that Slack is actually worse in many use cases.