When we say that a mailing list is "too noisy", what we mean is that we have to blast everyone's inbox for every little comment, like this one. There's no way on a mailing list to decide what conversations / topics you want to participate in and, conversely, who you want to engage or notify. It's like standing in the middle of a room and shouting at everyone. In my opinion, and many others we have talked to about this, it just doesn't yield good discourse. And Guillaume clearly pointed out that the mailing list archives lag so badly that it's difficult for anyone to jump into a conversation in a timely manner.
How switching to Zulip fixes this problem is that it's specifically designed to accommodate the kind of discourse we're trying to have. It's a modern and well-architected solution to address the shortcomings of mailing lists and other communication platforms. We've now had over a year of experience using Zulip for Asciidoctor and Antora and the results speak for themselves. It has totally changed the way we communicate. Multiple conversations are going on at once every single day, no one is stepping on anyone else's toes, and everyone who enters gets the information they are looking for quickly and efficiently. If you want to understand more, I recommend studying the case study I wrote about it: https://zulip.com/case-studies/asciidoctor/
I'll share that we were reluctant to adopt Zulip at first for Asciidoctor because it seemed like it would just add one more channel to check. Now, it's practically the only channel we use (especially for support), freeing up the issue tracker to focus on technical changes.
As for the concern about "increasing the amount of messages for the project", we desperately need more communication to happen. We've been riding along on fumes for several quarters because we can't find a practical, efficient way to communicate. I anticipate switching to Zulip is going to be the fuel we need to lift off.