If the mailing list server tampered with the message, everyone will get the false message. But if it is an encrypted group chat on matrix for example, you can 100% verify that the person posted that message (provided you've verified them, but even if you haven't, your logic of other people having verified it can be used).

As for archiving, you can archive mastodon, lemmy, bluesky, matrix,etc... as you said, if it was important, it can be made so for any open platform. Public archiving services will use APIs instead of mail clients, that's the only difference which shouldn't make a difference to users.

Even on the closed source platform twitter, there are public archives of things. Governments post official communication and records on there. Some governments are experimenting with Git for law publications. Nepal just voted an interim leader over discord!

This nostalgic thinking about mailing lists is incompatible with a technologist's mindset.