Slack is kind of my litmus test for whether someone understands what's wrong with FOSS. If you do not understand why Slack has grown so much vs. IRC, you don't get it.

User experience is a huge f'ing deal and FOSS doesn't care about user experience. There are two reasons for this.

One is that UI/UX work is not fun and so programmers generally must be paid to do it. This is an economic model problem and in some cases a management problem. Jamie Zawinsky observed this over a decade ago and not much has changed: https://www.jwz.org/doc/cadt.html

The second is that many in FOSS view mastery of arcane systems as a status symbol and have active contempt for any effort to make systems easier to use or more approachable. This is an elitism problem. FOSS is about freedom and equality among developers, but there's also a mentality among many (but not all) that looks down on non-technical people and resents any attempt to bridge that gap.

IRC lets people message each other and chat on channels, and Slack lets people message each other and chat on channels. The comparison ends there. Try getting a non-technical person on IRC vs. Slack and now try teaching them to really use IRC -- to transfer files, etc. Now try getting IRC working on a phone. Call me in a few days.

Slack is not just warmed-over IRC. All the apparently little things that make Slack so easy to use for so many people took a huge amount of work to implement. UI/UX is hard and painful and requires loads of testing, iteration, and attention to detail. Little things like the fact that when I edit a Slack message it changes everywhere at once reliably are things that took... oh god... I shudder to imagine how much pain.

I do believe that open source can overcome this problem, but step one is actually understanding it. So far almost nobody in FOSS gets it, so I'm not optimistic.

Edit: one more point about user experience...

It's not just about luxury and convenience. It's about cognitive load. Forcing people to learn endless amounts of arcane nonsense to do trivial things that have been done since the 70s is a waste of peoples' time.

In the information age information is cheap but attention is incredibly expensive. If something doesn't work instantly it's broken. If I have to spend time figuring it out it's broken. If I have to learn anything more than I absolutely need to learn to make it work it's broken. That's because there are a billion more new things to learn that are waiting in the queue and most of them are probably more important. Next, next, next!