Microsoft Teams is not designed to be sold to the same people as Slack.
Slack is sold bottom-up. Its a solid product. Its designed to get users to try it, like it and buy it when the trial expires.
Microsoft Teams is designed to be sold top-down. It is designed to be sold to Enterprises with Office 365 subscriptions, which is basically every Enterprise. If you are an Enterprise, and you want a chat app, now your 'default' option is MS Teams, which comes with Office 365.
MS doesn't even want you to try it unless you have Office 365. The complete opposite of Slack.
MS Teams doesn't have to be a better product than Slack. Heck, if its like most MS Software, it will be buggy for years. It just has to be 'not bad enough' that you don't have to buy Slack.