Replace mailinglist with discourse

you can read discourse via email (if you subscribe to all the mails in your preferences)

and you can even reply to threads via mail.

No, discourse does not replace the mailing list. There are a number of important things that are not possible with discourse (discussion of source code patches, for example), and we will certainly not force users to switch to a tool they are not comfortable with.

  1. you can reply to discourse threads via mail. IIRC nowadays you can even start new threads via email
  2. there is a mailinglist mode in discourse where you get a mail for every thread in the forums you subscribed too.

so in theory it is easy to replace the mailinglists with it.

It’s not happening.