You can opt-in with role-based permissions. Discord has been used by massive servers with hundreds of thousands of users, it's a bit involved to set up but it's not impossible. You can even automate a lot of it with bots.
You understand that an admin needing to set up a channel, a bot and a role is a bit more involved than any user just hitting create channel and inviting the relevant users/users searching and joining the public channel, right?
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.
