The main issue with that planet were lightnings, the storms, the ionization etc. As far as I recall, all other planets had breathable air even with their defected vaults. It might be so that Habitat 7 would require additional work (thus, unbreathable air up to your last battle against Archon). Considering we haven't taken down the local Architect, there's the possibility Architects repurpose Vaults after one of the terraforming phases is completed, who knows.
maybe more than one person could know but then again we shouldn't count him completely out. if we learn anything from playing mass effect its the dead don't always stay dead. he was willing to keep his wife alive and make his own kids think she was dead so i don't see why killing himself would be any different. plus if sam could alter the body like he says he can why couldn't he alter alec's lungs to allow him to breath the air long enough for help to arrive as well as save sara/scott. i mean clearly sam has more abilities than even we can imagine. all i am saying is if alec shows up in future games i wouldn't be surprised.
