Why should they support browsers that make dramatic, arbitrary changes on a whim? Chrome is the only browser that remains stable. Who knows what Firefox or Edge will look like next year?
Do you have proof that Chrome is the only "stable" browser? Is your standard for stability Chrome, meaning that by definition Chrome is stable and other browsers aren't if they aren't exactly like Chrome?
What does "stable" mean here? Chrome, Firefox, and Edge all take Web compatibility extremely seriously. They would not break Slack.
Look like? In what way does the interface of a browser changing make it more challenging to support as a website? Firefox looks a bit different now to a year ago with Quantum etc, and add-ons are very different, but I can't think of a single website that would be affected by those changes. Can you elaborate on what you're getting at here?
Why not have the whole world stay on ie? Nothing ever changed there!