While messaging apps have tended to be cyclical, I'd argue Slack has become much more than a simple messaging app at many large companies with their integrations and bots, and they are intrinsic to many teams' workflows (e.g. "ChatOps" is a real thing).

I see Slack as more akin to something like Excel, where people who really depend on it (in a way much more than person-to-person chat) go beserk if it's taken away.