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.
> The app is impossible to work with without a mouse.
It is, in fact, not.
I disagree that it's super difficult, but even if so, difficult is substantively different than impossible.
You, on the other hand, decided to pretend I said something I didn't so you could be indignant and tantrum at someone who... doesn't work on Slack and makes zero decisions about what to implement in the Slack app.
up and down work pretty well once focus is in the historical messages window rather than the edit window
this is on the current mac client