I have answered this like hundred times already, but okay 
Aluminium has lower density which makes it lighter but also softer. This means you need thicker aluminium sheets if you want to weld them or press in threaded bushings, so it's not necessarily a weight saving on its own.
Sentry is made as a self supporting structure because of that and it is the main factor that is saving space in Sentry. Note that whole chunks of the case come out so you can have free access for installation of each component (apart from AIO which is complex procedure and another topic).
Sentry uses welding to save space and make it look properly at the same time - with aluminium and rivets you can have one without the other - you can use rivets in an internal frame and it will be bigger or you can have rivets everywhere on the outside in fairly random spots in context of the aesthetic design, that are supporting the internal structure.
Weld points need to be covered by the paint/powder coating because they are really visible (you are melting and mixing the metal in the spot where it's welding) and thus even if we had aluminium case that is welded, we'd have to paint it over unless there's some kind of repeatable procedure to brush the weld spot to a point it's indistinguishable from the rest of the surface.
As for rusting - we are using galvanised steel, so with powder coating it's technically double protected against rust. If you scratch the paint, the rust won't start coming under the paint making it fall off in huge portions like you usually see on rusting steel pieces. Also if you somehow go through the galvanic coating but the scratch is narrow, this galvanic surface should still work against rusting from the sides of the scratch.
Obviously it's not indestructible, nothing is so if you hit the powder coating with something harder with sharper edge, then it'll scratch or chip-off, but while the first prototypes that were not painted and were made with normal steel are rusting like hell all over them, I don't remember seeing any rust on anything that was already painted and used for testing. Even those cases that I've been using personally, dragging around in the backpack back and forth on a bus did not rust where it were damaged.
As for the weight, well, once again it's a trade-off for getting better protection of your components if you are taking it with yourself when you travel.
With Sentry you can take it in anything that can fit it inside while with other aluminium cases you might have to go with something fully padded specific for the shape.
I actually didn't knew they started selling it. I've seen they failed to crowdfund it on Kickstarter. This design is neat, but flex PSU is a terrible idea IMO. Also as you said, the glass panel makes it completely different target than Sentry's.
