Good points, and it doesn't make Slack or anyone else look any better. They aren't doing what they do for the good of communication and long term retention. They are making themselve a single vendor point of failure on purpose, like so many companies have tried and will try to do with their customers.

I don't believe there is a dichotomy between IRC and Slack. In fact, Rocket Chat and Mattermost show that locally hosted platforms can be developed openly and serve a similar purpose.