Hi Elliott!
I’ve actually been thinking about this idea a lot recently. I’ve even proposed to move the international mailing list to the discourse for the same reasons you list here.
The reality of the mailing lists that I’ve observed is that all the people who actually legitimately use the mailing lists to discuss topics frequently also have an account on the discourse as well.
The mailing list can at times be difficult to parse what is happening in a conversation, especially on a single import thread, since the archives only show messages from a single month, while a discussion may last several. Additionally, you can only view one message at a time, further limiting readability.