Hi Sacha,

thanks for the link to the discussion, it's always good to get such
feedback from users!

I don't know if we can make it easier to customize package-archives
from the view the user get after M-x list-packages RET.

I suggest to try enhancing the discoverability "upstream", by updating
the default Emacs menu bar.

For now, package is present in two menus: [Options] and [Help].

  [Options] contains "Manage Emacs Packages" which M-x list-packages,
  so it does not solve the problem you are insisting on.

  [Help] contains "Search Built-in Packages" and "Find extra packages"
  which leads to an Info page, not directly to the customize menu.

I suggest the following patch, which moves "Manage Emacs Packages"
from the [Options] menu to the [Tools] menu and add these subitems:

- List packages
- Customize package archives (<= the option you want to promote)
- Customize the package interface

I think package is more a "tool" than an "option", so it makes sense
to move it there.  Also, as you mention, just listing packages leaves
aside the possibility to customize the package sources, which is what
most users may be looking for.

I think "Search Built-in Packages" could also be moved to this new
menu, while "Find extra packages" could stay in the [Help] menu,
because -- but I don't have a strong opinion on those.

I attach a (rough) patch for this idea, let me know if it sounds
like an improvement to you!

Best,

-- 
 Bastien