Are you saying that you think Slack's primary challenge is font rendering?

No, it’s providing a web service to do something that would work (and used to work) much better without it.

Their primary challenge is making money, and that part probably is better as a web service, but it makes it worse from technical point of view. Just like with hypothetical FaaS - worse technically, but you can have adds.

I'll bite, what exactly worked much better than Slack without the web?

Pretty much anything, starting from IRC - it offered more functionality, from logging to scripting, but the general idea is the same.