It is better than E-mail for instant and group communication, it is better in everything except audio/video calling that Skype. SMS and phone calls are not even in the same ballpark in terms of solving group communication problems.

Disregarding the technical side of things, Slack is a great tool and it is quite understandable why it stuck: people like it.

Well, it replaces all those things when you want async messaging with a decent UX.

I'm not sure how serious to take you when you think Skype is better.

Especially when Skype in this context probably means "Skype for Business". Oh, that user just went offline and Skype wasn't aware? Well, you'll be informed, nicely, that the message couldn't be delivered. Will it be delivered? Oh, of course not, this is 2018, we don't do that.

Or you're signed into Skype for Business on multiple devices. You wouldn't expect to receive instant messages on all your devices at the same time, would you? Because that's really asking for a bit much in 2018. We're not wizards, you know.

And planes didn't replace boats, trains, or automobiles.

Baffling that so many people still use it as yet another mode of transportation.

You are in one mode of transport at a time, but your phone can receive multiple forms of communication at a time. All of them with their own notifications, badge icons, and alert sounds, and behind those, some person demanding your attention and only the most minimal cultural etiquette about when to use which and when, if any etiquette is present and pervasive enough through your social and business circles at all.