I agree that there’s a need for communication / collaboration tools that allow easy work across organizational boundaries. MS Teams permissions to allow for outside collaborators is painfully complex.
But Slack Connect, while better than Teams, still doesn’t get the job done well. So Slack’s so called “competitive edge” isn’t that competitive.
For outside collaboration / communication - I like AirSend (www.airsend.io). It’s a lot simpler than Teams or Slack - less integrations, but perfect for people who work across organizational boundaries on a regular basis.