Let's to summarize the discussion:

1) There are people who want the Community discussion in the forum.
2) There are people who want the Community discussion in the mailing list.

It's an endless discussion. Some prefer forum and some prefer mailing list. So, the forum shouldn't be closed, and the mailing list shouldn't be closed.

Solutions:

1) Leave everything as is. Very easy.

2) Synchronize the forum with the mailing list and viceversa.
This is very different to the possibility to use the email to reply/create threads here.
It should be a complete sync, for example: posts to the mailing list are visible here, and posts here are visible in the mailing list.

I'm completely for the solution 2, of course it's the most complicated technically, but the best solutions generally are the most complicated.
With a first look, there are some options like m2f or NNTP gateway

The 3) solution is to be discussing this until that the climate change ends the world.