People use slack because they use it at work and don't want to have two different message clients and want to consolidate. Free software is not meeting the needs of corporate users and has failed them, so they are using Slack or HipChat instead. Your dogmatic stance on "I am completely opposed to editing messages after they are sent (chat is append only!)" is a good example of how open source is failing corporate user. I typo things all the time and want to be able to edit them, and don't care about your philosophical objections.

You can try to bludgeon and guilt people into using open source tools, but the problem is fundamentally that the tools that are Open Source are /failing/ a set of users. If you fixed the tools and ran HipChat and Slack out of business with Open Source competition, we wouldn't even be having this discussion. Lecturing people about Freedom(tm) and "Devsplaining" to users why they don't want the features they tell you they want isn't going to work.

If you want people to get off of Slack write an Open Source alternative that Slack users will freely choose to use instead. Compete in the market and win -- or else you're just pissing into the wind.