In regards to the Reddit redesign, I think the people complaining that "they ignore their users" are stuck inside a bit of a bubble.
I have a lot of friends who recently discovered the site, and they much prefer the redesign to the old one.
I forgot where I read it, but I vaguely remember stats backing this up, or at least increase in engagement or a reduction in churn for new users following the redesign.
Something like 50% of Reddit users are on mobile, for which the redesign is targeted (I believe)
Re: Slack, I know many non-techies who struggle with markdown and would welcome a WYSIWYG editor with open arms and wide smiles.