Super small footprint, I can do everything with the keyboard, I can script it, I can run it on a server in SSH, I can customize it any way I want.
Meanwhile on my large screen I can barely see 10 messages in the backlog of the signal desktop client because it feels the need to emulate an SMS UI (you can even select if you want it to emulate the Android or iOS UI, how cool is that?), I still haven't figured out if you can create groups in the desktop interface (given the very small number of options available I don't see how I could miss it, maybe it's simply not possible). The input box has english spell checking enabled by default which is pretty distracting when I'm typing an other language since everything gets underlined, I haven't found a way to disable it or change the language.
The other day I had to use the Discord webapp for something, what a joke. On my pre-quantum Firefox it was laggy as hell, there was a noticeable delay sometimes while I was typing text. Maybe you'll blame Firefox for being too slow but we're talking about an actual text chat here.
It does look better than a terminal window though, I grant you that. Maybe I should print it and frame it somewhere in my office.
9 times out of 10, a github project fixes my bug report very quickly.
Half the time I'll send an email to a closed source project and never hear back about it.
FOSS is also doing well in an era where proprietary clients mean crappy electron clients. That's a low bar to beat.
I've yet to see FLOSS crowd come up with a remotely useful client for anything. Event in the days of much simpler APIs and apps like IRC FOSS apps were universally meh.