> Now we could have some kind of system whereby excess icons get dumped into a … menu, but that defeats the entire purpose since they’re no longer on the screen all the time!
This is just not true. The purpose is not necessarily to have the icon on the screen all the time. If the menu doesn't exist, the buttons don't exist either, and plenty of applications are built assuming that you have access to their bar icons while they're running.
If the icon is missing there is _literally_ no other way to use the relevant functionality. If the icon is hidden in a menu, you can use the functionality, it's just an extra click.