To have all the features you get for free out of slack, you have to do a lot of work and maintenance to get out of IRC and mailing lists.
Mailing lists by themselves are terrible, because there's no history. You can't scroll up to find context for the conversation. You can get public archives, but then where you actually use the mailing list and where you go to look at history are completely separate, which is a terrible UX. Google groups mostly fixes this because they're really just a forum that supports emails.
IRC is terrible because everyone has their own client, so you never actually know what someone will see when you post something. Post a link, - will it get auto-expanded? How will it look? Will it get auto-linked? If you @ someone, will they get notified? if so, how visibly?
IRC also has the multi-login problem - it gets confused if I log in from my phone and my laptop. To get a persistent history of IRC requires running an external service which is way beyond most users.
Slack has mindshare, which leads to having a ton of integrations with other platforms, which just multiplies its effectiveness.
I 100% agree that I wish they offered a publicly visible option, so anyone can view without logging in. I also wish they natively supported self-registration.
I have to echo another commenter on here. If you want the best of all worlds, a forum is what you're looking for. Publicly visible, easily linkable, long form, asychronous communication. Whether that's google groups or something else is up to you.