Core can look if the system has any nodes at all , and only emit the welcome
message if there are no nodes (not nodes promoted/published).

The message isn't in Drupal 7 any more, or at least, not at /node, it's been replaced by a short message inviting you to add some content.

The real answer to this is the other thread on this list at the moment - views in core, so that the default front page (and default taxonomy, forum, blog and other listings) is just a view which you can enable, disable or modify.

Nat