Good clarification -- when I talk about change, I mean a workflow change. XKCD aside, I don't believe that every code change breaks someone's workflow, or at least I believe that some code changes break workflows in such a minimal way that none of your users will care.

To expand on your point, how many bugs and crashes do we tolerate in software simply because rebuilding features has priority over refining features or handling more edge-cases?

Slack's new interface isn't just different, it's buggier. Stuff like copy-paste is broken. So it's not just that we have to adapt to a new workflow, we're also accepting a lower-quality piece of software that is less reliable than what we had before. And in theory a refactor or rewrite might be so valuable that I could tolerate that, but I don't see that value here.