If people cared about looks or features, Slack certainly wouldn’t come out on top.

No, what people care about are primarily network effects. They'll use whatever someone in the company decided to use (frequently Slack because it’s always been Slack since someone back in 2013 decided it was the hip choice, before it even had a desktop client), and they’ll put up with whatever dogshit client they’re served, because it’s out of their hands.

Slack has great features and awesome integrations with almost anything you can think of. This is its value and this is why people like it.

In the last two companies I worked in it was us low level employees which were begging IT for Slack, until they reluctantly accepted (because of costs and of fears about hosting confidential chats on 3rd parties). Nobody was begging for LimeChat or IRC.

And what about Discord? It's almost exclusively used by people in their homes, there is no company pushing it down their throats.

There are network effects outside of companies "pushing [software] down [employees'] throats." Consider, for instance, that large groups of gamers already use Discord?

Some of us would love to do it, but job and customers aren't there.