Sorry to be a party-pooper here, but I hear a whole lot of rationalization going on in this discussion trying to support the upside-down image on the ground glass as being "natural".
I don't buy it.
No way is it natural. While I'm sure that those who have viewed such upside-down images hundreds or thousands of times have learned to accommodate it, I don't think anyone can convince me that it's as natural as viewing the scene right-side-up.
I wish it weren't so, and that my ground glass showed the scene the way I see it with my eyes. But it is, so like everyone else I learn to live with it. But please, get real: don't let's insist that it's easy or "natural".
Actually the human brain is very accommodating when it comes to vision and spatial direction, so an inverted image can be reconciled quite easily if it is presented enough times, or for a long enough duration. In other words it can be learned. Once learned, it is for all intents and purposes "natural" to one who has acquired the skill. Look up the experiments of Dalibor Vesely if you find this interesting.