People choose keyboard-centric workflows:
a) For efficiency (if it takes a few milliseconds to press a key, why would you even reach for the mouse?)
b) For accessibility
Your point is not helpful for any of these cases. Asking to try to use Slack App with a blindfold might be a bit extreme, what if I suggest you to try to use it for a few hours without touching the mouse? How's that for a challenge? Try that and maybe then you'd feel how it supposed "to get you there" for people with disabilities.