There's probably a lot more that goes into this than you realise.
You say you could just pop the HDD in an easily reachable bay on the side of an iMac... Well... you couldn't. Sorry. It just wouldn't fit. With the depth of the panel there's no room, the optical drive doesn't have any space around it and they're 9mm units I think? There's really not a lot of spare space in an iMac - or rather there are lots of very small bits of spare space!
If you want the designs to be so sleek, they pretty much have to be that way, and if you want easily replaceable HDDs, the machine just has to be uglier and fatter unfortunately.
There's a knack to getting iMac screens back on without dust, but you're absolutely right it's a big pain.
I've serviced quite a lot of Macs, and the amount of times I've got the screen back on first time with no dust?
One! One time! We've got a special magic roller and everything.
As for MBA RAM... there's just no room, honestly. It is tight as **** in there, the logic board is tiny. There's a keyboard on top of there that's also recessed into the top case, remember. There's just no space. RAM slots are actually quite bulky when you're talking about a machine as tiny as an Air. It's nearly all batteries and once you take into account the wall thickness of the chassis, there's nothing spare.
Also bear in mind that the majority of their customers don't care. We do, but we're great big geeks, most people aren't fussed.
I'm with you btw on wanting upgradeability, I'm sizing up my next computer and I think I will get a Pro and put in an SSD and optibay.
On a 15" rMBP (haven't opened up any 13s yet), the logic board is so small and thin it feels like it's going to snap when you fit it. There's a fan cutout on one side and the logic board goes down to about 7mm x 1mm at one pint. Really flimsy.