No you don't. It's per "team". Assuming that's what you meant: how many teams do you honestly use? Sure, it's not ideal for a bunch of little communities. As you said, Discord is good for that.
For a business, I haven't seen anything that even compares with Slack. I'm not sure what you consider a "normal user" (I'd argue that there is no such thing). If you mean non-technical users, then there are plenty of non-technical users in a business that need to communicate. For most businesses, they outnumber the technical people and rely on good communication tools more.
For work, Discord would be terrible for me since I can't have more than one username (to my knowledge). I want to keep my personal chats/username/status separate from my work ones.
Different people need different solutions.
BTW I'm focusing in on the business use case as that seems to be who Slack targets. They are trying to sell to people that have used Skype and Lync (or whatever they are calling it this week) and hated it.
EDIT: also, building a decent chat app is a very difficult problem. I imagine they will use that money to maintain the various clients they have and try to keep adding communication features (as they have been doing).