Not quite your main point (which I agree with) but comparing surface area of orbits to the surface of the earth doesn't really help understand the carrying capacity of orbits.
There are so many constraints on objects in orbit, the degrees of freedom so limited, that carrying capacity is much smaller than you might suspect (though probably larger than our earth-bound intuition).
Just for starters, each item in orbit traces out a path (an orbit!) that can not intersect with the path of any other object, without carefully considering phasing to make sure there are no collisions.
We keep items separated by kms (hopefully!) because there is too much variation and uncertainty in orbits.
You can use different altitude shells to fit more items in, but they take longer to orbit and increase latency, so you have to be able to deal with that.
Based on degrees of freedom, orbits are far closer to 2 dimensional than 3 dimensional.