There are quite a number of other differences between the 1DX and the 5DIII, but none of them are any more significant to real-world shooting than the noise difference we see here.
Depends on your definition - if you're talking only IQ, fine. But...better sealing and 250K more shutter actuations seem like they benefit real world shooting, not to mention the improved metering.
The 5DIII already has plenty of sealing, more than enough for most camera owners. Not many people will find themselves in situations where the 1DX is adequately sealed but the 5DIII isn't. Some, but not many.
And not many people will wear out a 5DIII shutter. Those few who will are going to be CPS members and aren't going to notice the $250 or whatever fee to repair the camera plus the day or two it takes to get a loaner while the one is in for repair. They're not going to notice, of course, because it's one of many backup cameras they already have for exactly that sort of contingency.
If the metering on the 5DIII were problematic, you'd have a point...but, again, the 1DX is better while the 5DIII is just fine.
Just to reiterate: I'm not trying to claim that there's no difference between the 5DIII and the 1DX. Quite the contrary. My point is that, except for the fact that the 1DX is an unbelievable speed demon monster and the 5DIII is merely not pokey, the differences are fairly minor, especially in the real world.
And, to continue the theme: most people don't need an unbelievable speed demon monster and so don't need to spend the significantly extra money on the 1DX. That's especially since, in the film days, the 5DIII's 6 FPS would have been considered reasonably fast, and even (barely) adequate for pro sports. But you'd probably lose your job if you tried to cover the Olympics for SI
with a 5DIII instead of a 1DX, and rightly so, and those're exactly the sorts of people who should be snapping up the 1DX as fast as their credit cards clear the bank.