Is 16B really too high of a valuation when Atlassian is double the market cap at 32B? Both have similar products (collaboration/productivity) and are extremely sticky.
Atlassian has a wider suite of products. I would say HipChat is the Slack alternative. Slack has nothing like Jira, BitBucket, or Trello. And Atlassian is more developer centric whereas Slack is monetizing enterprise users.
Actually hipchat was quite nice to use, fast and responsive for awhile, and I loved some of their built in emoticons. I think they didn't really maintain it and keep it up to date with the competition.
Atlassian products are more mission-critical. Most companies actually need some tool that does what Jira or Bitbucket does. They don't really need Slack.
I've worked at companies both with and without Slack. Something like Slack might be mission critical for a remote-first company, but comparing the two situations, I've actually noticed better communication without Slack than with it.
We use slack at the three companies I currently work on and it's mostly a distraction and a place to quickly share files. It's good for some shared notifications. My most active slack is one I have with friends.