Well, I agree with pretty much everything you listed there save 2...I'd just go with the much simpler explanation that they had their chance at exploration, and didn't see anything they liked nearly as much as Krypton, so decided to stay mostly on that one planet...and then got caught by their own arrogance, believing they could handle anything. That also leaves open the option should someone choose to take it later to bring in a lost Kryptonian colony from back in the day. Kryptonian gods...just have never seemed to worried about mortals, Kryptonian or otherwise.