"And I use two cameras more often than not."
So do I. 
I almost always have my 1D Mk IV and 1Ds Mk III. I carrry one with a hand strap and the other is on my Black Rapid.
"At about twice the price of a 5D3, the 1DX had better be at least twice as durable"
A camera, or lens for that matter, is a sum of it's parts and not just one spec or two. Althought the product that a 5D produces may be just as good as a 1Dx, the 5D Mk III is not a 1 series in any sense of the word.
"And they use them and use them for years and years"
There are guys still using their 1D Mk II n bodies.
When people bring up trying to compare a 1 series to any other camera, I always tell them, in the end, once you go 1 series you will never go back. The old saying between 1 series owners, "You can beat somebody to death with a 1 series camera and then take the crime scene photos with it!? Ya gotta love it.............
