"There exist other edge cases, so let's change the subject."
There are a lot of tools that should work better offline. But a remote collaboration tool is not a good example of that. I've done work on secure private P2P messaging systems, and when you want that feature set, you really want that feature set.
But when I use Slack, it's because I don't want to have to be in the same building as all the people I'm working with. There is no way for this to gracefully degrade with a network partition. My argument is that central coordination and audit (with Slack) is not an edge case, it's the primary case.