I don't understand your point - Slack can't scale for extremely large teams well. I've watched the desktop and web app grind to a halt on the first load of a channel because it's still loading in channel history/offline messages. Beyond that, when trying to tab-complete a nickname this similarly almost freezes the desktop app. This has been my experience as part of a team on Slack that has 4000+ users. These are client-side problems, really - but I don't see how you can say Slack scales better when all of its [great] features come at a cost that limit how large a team can be. The larger the team, the more noticeable it becomes.

The history should load in progressively as needed. Switching channels within the same team should be instant. I don't know how you fail at tab-completing nicknames and commands, but somehow they made that slow. I get the feeling rendering is not on a separate thread in the desktop app.

Anyway, you're right that not everyone needs to be on 7-9 networks with the ability to talk to everyone among those. Slack is great for teams of like 500 people and less (imo).