In their defence (not the defence of the WYSIWYG box itself) - they quite likely have done lots of user testing that has shown clear desirability, improved value to user, improved usability, etc etc. You have to remember Slack's target persona is probably no longer the Engineer (If it ever was) - it's more likely a much less tech-savvy employee who finds WYSIWYG editors very handy to create rich text inputs. I guess…
IMHO they missed the fact that power users would have preferred the old behavior and didn't provide the opportunity to switch between the two behaviors
I get little pangs of dread a split second before I realize I'm about to have to spend 10-30 seconds figuring out new formatting since it shipped.
And as a response, I'm less reluctant to use it, meaning my messages are less detailed, meaning longer conversation, less precision.
It's a "convenience" for non-power users, on a feature only used by powerusers