That has nothing to do with Slack-like apps, and everything to do with Slack.
> Go talk to the hundreds of FOSS communities that use IRC. they have all fixed the persistence / search problem
I am part of many FOSS communities on IRC. None of them have "fixed" that problem. There might be one or two that archive the channel somewhere. That is no substitute for real in-app persistence and search.
There is a reason slack has a problem with that - scaling is hard.
There is also the advantage of not having silos - if I have a problem with a dependant library I can "/j #libname" and ask a question, instead of searching for what slack, or slack like tool they use, signing up, installing whatever app is needed to access it, and asking the question (and remembering what app they used, so I can keep it open for issues that run over a few days.)
Detachable screen sessions make up somewhat for it, but they're still pretty limited (you might have /joined the channel only later), and it requires a personal service running, that must be attended to & etc.