The market simply optimizes for whatever is easiest for a ceo to install and maintain.
Seriously, what even is this comment chain getting at? If you work somewhere where always-on is a hard requirement, it doesn't matter what messaging system is used. Any place that would fire you for muting notifications would just as easily fire you for turning off email notifications.
In short: Let's stop blaming the damn tool for something that is the fault of the humans using it.
Slack is no different other than name recognition and size.
Slack is focused on business.
Slack doesn't have some voodoo set of features that make it more problematic than Hipchat or Teams or Stride or Mattermost or (nn programs elided).
If your org sucks, your communication within that org will suck too. Let's not burn down the concept of instant messaging in the name of sucky orgs.