Wasn't the target audience of Slack, at least initially, tech-savvy IT teams? If they take away plain text and markdown -- or make them harder to use -- they are alienating their early adopters.
Not sure what their user base looks like now, though.
I've seen it used in some surprising places. Last time I had my contact prescription checked, the staff used Slack to communicate between the doctor/assistants in room and the front desk.
Not sure what their user base looks like now, though.