Webmail clients like Gmail just butcher the mails.

Too true. I tried using mailinglists with gmail nearly a decade ago and regretted it.

I believe people hate email workflow just because of how badly its interface is designed.

And the amount of tools you need to know of in order to have a bearable experience. b4, lei, patchwork, notmuch, aerc, ... sounds like a lot of work and knowledge needed just to be able to use a mailinglist. Source forges have an intuitive interface that allow even beginners to contribute without setting up a bunch of tools.

IMO any project looking for contributors and using mailing-lists is either stuck in their ways, targeting a specific group of people, or both. Mailing lists don't bring the boys to the yard. Hopefully the linux kernel maintainers learn this some day.