In reply to just some thoughts...

I daresay Amazon has done more than it's fair share of testing in deciding that menu structure. Can you imagine editing a large content site, similar to Amazon, in that tiny menu editor? Heck, even just "mega menus" can be simple and contain only a few dozen items overall, visually in the admin they have 3 levels which is clunky enough to use at the moment let alone in that limited space.