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".