Yeah honestly I don't see why it couldn't replace Slack, it is a very similar product but I feel like by marketing it primarily to gamers they are positioning themselves in a far less profitable market than Slack is.
https://feedback.discordapp.com/forums/326712-discord-dream-...
https://support.discordapp.com/hc/en-us/articles/219070107-S...
Anecdata:
https://news.ycombinator.com/item?id=14870899
https://news.ycombinator.com/item?id=14865292
Some people consider this a positive.
I see several reasons.
1. It's impossible to not be in a public channel. Yes, you can mute it, but you can't ever leave it. In a large company with thousands of channels, this creates cognitive overhead.
2. You can create private channels, but this requires the ability to assign permissions. This has two problems - one, if you have a lot of teams that need to have private rooms you're going to spend a lot of time juggling permissions, and two, if you need a quick ad-hoc room that you don't want littering the general list, you need to be able to assign permissions.
Services like Ventrilo, Teamspeak etc required a lot of setup/ self hosting and were only really strong for voice. Skype didn't organise well around how people game.
When I come back to WoW a few years ago a lot of people were trying to run groups on Skype which was a horrible experience of adding randoms and joining group calls, in additional to all the other downsides of Skype. Discord is a perfect replacement for that.