Slack is an absolutely horrible name, especially for work-related purposes; it doesn't reflect messaging but laziness and productivity waste.

Which is exactly the point of the name, isn't it? The name makes you think of leisure time, but in fact you are still working and discussing work-related things while using it.

You haven't seen most people use slack then

ICQ has a quirky meaning to it,once you get it,it's like what you use when you seek someone (not really but the mental association is there). Slack? Not sure, it's popular in tech but I've only used it when I have no other choice but I associated it with slacking off (as in when you want to talk informally relax and talk non-work) or having slack (break or headroom),so when someone says let's chat on Slack, I think of a fun interaction that is more akin to taking a break.

But to your point, the only good thing about Skype for example is few syllables and messaging related graphics/logo for me.