1. Moderation tools on mailing lists are extremely rudimentary. A healthy community _needs_ moderation of some sort.
2. A lot of tooling is necessary to make a good mailing list experience. I would expect an inclusive community mailing list to have a web interface for posting and searching, and a separate interface for searching.
3. There's no documentation on setting up email clients to use mailing lists and it isn't a trivial/intuitive process. The email spam problem has made the registration process non-intuitive and often drops emails to/from the list.
4. There's very little documentation on setting up your email client to use a list properly. What are best practices for organizing your inbox around a mailing list? How do you reply to a list? How do you quote a previous response?
5. There's no easy basic flow for folks to get started with. The power of more "modern" approaches is that the default web interface is highly opinionated. This makes it easy to join and easy to participate. When there's no basic flow to use, folks never get to the basics before they get to a point of wanting to customize their flow.
I think a blessed mailing list flow using a webmail client, a web newsgroup/BBS interface (which doesn't exist now sadly), or a blessed email client workflow would go a long way to making mailing lists accessible. Unfortunately because mailing lists are only popular among tech people these days, I doubt you'll see much work on this.
Like you, I much prefer mailing lists over any sort of web forum (discord included). I hate Swift’s forums. I like the Python mailing list. One idea the reasons is that in the mailing list, I get to pick my tool (my mail client), and you can pick yours. We’re leveraging an already strong communication ecosystem of tools.