Yeah - once again I get that a lot among people who apparently don't understand what cooling is.
The bottom of a Mac no longer gets hot for a very good reason: The aluminium shell is *not* being used to dissipate heat (at least no more than a plastic shell on other notebooks for example), despite the fact that your average ignoramus / Apple user equates aluminium to heat transfer. You know why? Pretty obvious actually - if Apple used the case as a heatsink, instant lawsuits.
It is, as I mentioned, ultimately purely as a tactile salve for the type of guy/girl who typically opts for Apple, which is seen as an engineering aspect by English lit majors and unsuccessful musicians who're writing about tech. And part of the cooling issue I have with Mombooks is not just the fan profile, the way the fans work, or the way the air is directed within the system - part of the issue is due to the effort they make to keep heat away from the casing.
I don't think anyone said it was. However again, just because Apple says machining something out of a solid block is the nuts, doesn't mean it actually means much IRL.
If we want to get into machining magnesium, relatively speaking yes - but only in terms of time - which obviously translates to money. But again, it doesn't make any sense to machine (beyond some degree of finishing) magnesium in this usage scenario (laptops) when a specific, more efficient process exists for it.
Whatever's usually said, the goalposts are usually shifted eventually to 'this is what I like, and what I like is what's most important' - so sounds right.