Wasted time and money. Time you can never get back.
Using the same logic, 5DIII owners are also wasting more time and money shooting at "only" 6 fps vs 12 fps of the 1DX; I mean the discussion revolves around action shooting here (6D AF vs 5DII AF), where fps does matter. I mean, theoretically, you can get your desired shot much faster with a 1DX
Plus, if, for some reason, you felt the need to upgrade from a 5DIII to a 1DX, you'd actually lose more money selling your 5DIII than someone else on selling a 6D when upgrading to 5DIII.
It always comes down to what your needs are. Me, having a 6D and 70D, I'm not "bugging" Rebel owners about the shortcomings of their cameras. If I wanted to do that, I could start with my wife (not wise) (using a T4i)
It's perfectly fine for me to accept that to satisfy their needs that's the tool they can smartly afford, there isn't anything wrong with that. If that wasn't true, wouldn't every beginner buy a 1DX as their first camera?
I'm perfectly happy with my 6D but that doesn't mean I wouldn't like to have a 1DX or a 5DIII, I totally admit they are better. But even buying a 5DIII would mean that I'd have to sell probably 2 of my lenses (such as my Sigma 70-200 2.8 OS and another). My photography currently is more well-equipped and well-rounded with the 6D and those lenses in my possession than with a 5DIII and not owning those "important" lenses.