[icedchai]

If you use a Wiki for everything, no. However, that is unlikely. You'll have to search more than one place anyway. Making Slack your primary store of institutional knowledge is unwise. Chats are full of garbage, random discussions, old / bad decisions, etc.

[skohan]

And there's no guarantee that slack will exist forever.

[kemitche]

No guarantee that Atlassian Wikis will last forever either, but I'm certain that's the wiki 90% will be thinking of when "use a wiki" is suggested.

Backing up your knowledge store is a very separate task from your day-to-day use of it. It's just as possible to back-up your company's slack messages as it is to back-up the company's wiki.