I think you're overlooking the quality measure of the proprietary garbage as it evolves over time. Slack, in comparison to say Lync, or Skype has made huge moves towards better usability. But for a company like Slack determined to maintain market share, to control the user experience for all its users (whether they want to be users or not) is repeating the same mistakes, not necessarily the corporations signing contracts and writing cheques.
Slack is not the end solution. It's a step in the direction to being less terrible in the realm of corporate communication, and will be replaced by something else that does better in the future. For now it'll hold on whilst it still can.
I disagree - in a lot of corporate environments, it once snuck in as shadow IT to work around the then incumbent, infosec certified approved solution of less usable.
Slack has become the new corporate mandated unusable messaging application.
Slack is not the end solution. It's a step in the direction to being less terrible in the realm of corporate communication, and will be replaced by something else that does better in the future. For now it'll hold on whilst it still can.