Just to clear something up , because I see people in slack saying goodbye, or "I cant reply right now I have something to do" . Slack is not just a real time chat, it keeps messages inside a history of 10 thousand messages. So that means you can log in Slack at any time and not lose a message. There is no reason for you to be online all time, no reason to say goodbye, goodmorning , goodafternoon. You can come and go as you please the exact same way as the mailing list.
The problem arises when we exceed 10k messages, the old ones get lost and I think having a place to store them is a good idea.
Also what is important is in the eye of the beholder, for example I dont care about any discussion about web development , its just does not interest me or database coding or many other things. I have learned to filter out the messages I dont care about in the mailing list and just reject them. Slack is same story, I quickly glance through its history and I can search the messages that interest me using the search bar , I can star messages that I want to keep, and I can comment on existing messages / code snippets which create a thread about that message.
In the end its impossible to keep the community in one place but to have a central hub that collects all the little gems can be quite useful.