I've never used Slack but in Microsoft Teams there are Teams and Channels and they pretty much map 1:1 to Discord's Servers and Text Channels.

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.

Teams is garbage compared to Discord or even Slack imo. Complete garbage UI.

This is my biggest complaint about Discord, and the reason I rarely check the few servers I'm on. There are three channels out of 80 I'm interested in. Just let me leave them!

You can mute channels and hide them, or mute sections, or mute chanells and collapse the sections they are in. There are lots of ways to get channels you don't want out of your face.

This isn't scalable. My employer has many many thousands of channels on our slack instance. Opt in is so much better than opt out at that level

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.