Teams is free as part of an Office 365 subscription (just like Sharepoint and all the other crap they bundle with it), which is what definitely makes it an existential threat to Slack, because now Slack is competing with Microsoft's very strong Office monopoly.
The fact that you can create a group with some channels, add your files there and edit them together, have a wiki, have a (not very good) kanban board etc. makes it a pretty complete experience. Some of the tools they release needs more polish and some tools are by themselves much worse than stand alone competitors but having everything in one is pretty nice.
Then there is sharepoint, oh, and onedrive, and they all come in the same package so everyone uses something else.