Traditionally offline message caching would be the responsibility of a bouncer, or something like memoserv. There is an interest to make this responsibility part of the server in IRCv3, though.

IRC scales much, much better than Slack. Unreal or InspIRCd or Charybdis whatever - Slack fails miserably for hundreds of thousands of users in one "team"/network.

I do agree administering/configuring IRC servers has to get nicer. I personally want an IRC server that exposes a REST API for online configuration (no rehashing of a config file).

99.9% of Slack users don't give a shit about hundreds of thousands of users in on network. Slack scales better than IRC at anything other than absurd rates.

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).