Congrats to them. I used to be an avid user (loved the way some integrations could help productivity), but since then moved on to Discord, which feels superior in every aspect, especially since it's free. I'm not sure how I feel about the valuation. I'd like to see how the user base, especially the ratio of paying users, has been growing. It feels likely that most of the companies that would have been easy to convert…
I do worry about Slack's pricing since there is a vast chasm between their free plan and their paid plans. I use Slack to run some open source and hobbyist communities with thousands of members and if for any reason we were forced to switch to a paid plan (at $x per user) we'd be forced to go elsewhere immediately.