I completly agree.
I think the best way currently to try to place some quality answers on the web, if someone references the TYPO3 documentation with a small comment in StackOverflow. That is straightforward, and navigates the people to the documentation. There are a lot of example, a lot of tutorial and it is perfect to learn from it.
I understand the concerns about Slack, but I think this is just a different level of communication. You have a problem, someone tries to help, you discuss it, there is really a dialog between two or more people. Just like asking your collegue next to you.
Mailinglist was a main tool to directly notify peoples with the same intrest and maybe start a conversation about a problem. (My personal opinion: the worst thing when I find such a years old mailing thread in google and had to keep reading and paging and reading and paging... Nowdays I simply ignore them and pray not to get any of those in the first 5 results)
Forums could work just like StackOverflow with a discuss feature on it. So you can ask your question, someone can ask back, you can quotate his answer and extend your question, and so on. This results again a long thread to read and cost time to evaluate if it is relevant at all or a completly different usecase.
The main point, that it provides a quick way to evaluate a question and its accepted answer. (And the way between the solution and the question is totally irrelevant)
If the goal is to discuss something quickly, active, Slack perfect for this task.
And if something really should be discussed, maybe not quickly, maybe not so active like on a chat, because people needs opinions, this site is perfect. But these topics are not really in the "I have a problem/ support" topic.
I personally don't see, in which way could a forum increase the current quality of the given answers or the connection to the community.
But to the original question, the idea of getting rid of these channels is a good idea. I would think about if a Discourse necessary or not.
Would be a nice experiment to shut down all of them, and see what happens.