I don't think this is true. You can't really join/leave channels on Discord (yes I'm aware you can use roles to give permissions but it's not the same).
You cannot leave individual channels of a Team though you can mute them and you have even less permissions than Discord.
So maybe it isn't a 1:1 replacement for Slack but it certainly could compete in the field.
EDIT: Another example of discord's permission system being a poor match for the business use case is pinning messages to the channel is the same permission as deleting other user's messages. Additionally, the channel per server limit is 500.