I think the success of Slack is that it's so easy to set up a team. It feels like Trello: you are up and running in a minute. As a freelancer I got a list of Slack teams and it is just very easy to switch.

But like you said: that's about it. It is slow, the text input box is horrible and most of the time you loose track of conversations.

What I lately notice is that teams start to use WhatsApp groups as Slack replacement. To me that shows that Slack is just a messenger.