Slack thought right; they did sell to businesses. Microsoft came later to the game, leveraging their size to both undercut them (with a feature rich free plan), and offer an integrated suite with the productivity tools that enterprises are already using (with Microsoft 365).

Which is probably one reason why Slack is looking to sell to Salesforce.

My point being I don't think Slack's positioning was "chat but prettier". The fact Teams is more dominant now seems more to do with bundling.