Snapchat was in a similar situation, and four or so years ago when they turned down Zuck's ~$5B offer everyone was saying what a silly mistake it was not to sell. How could Snapchat possibly make money? And think of all the disheartened employees whose shares would've been worth something.
Turns out there's a market for ubiquitous software. Slack seems similar.
Another datapoint: Look how well the acquisition of Oculus turned out. Palmer now has money, which would've come eventually. He no longer controls Oculus, and hence can't impact the world. And in a twist of fate, Palmer has attracted public annoyance due to some political decisions, so he went from favorite underdog to just another wealthy person.
For ambitious people, this can sometimes be a worse fate than not being rich. Ideas require opportunity and timing, and opportunities like Oculus, Snapchat, and Slack are rare.
$9B is nice. But are you sure you wouldn't rather affect the world?