Yes... the incremental cost....
I'm going to buy a 5D3 for $3000..... Canon comes out with a new $5000 camera that I like more and am willing to pay for..... It's not just Canon selling a $5000 camera, it cost them sales of a $3000 camera, so the net gain is an extra $2000 in sales from that $5000 camera....
I'm sure that thier business planners have considered this,.
The flip of this is just as important and probably even more so. If they don't produce the $5000 camera that you like more but someone else does they may be out any camera sale as well as associated lens, flash, and battery sales. For most people once they start in a direction it's hard to get them back. Canon would have to produce something so spectacular that you're willing to forgive and forget and re-invest in their
Look in the toothpaste isle, Crest has been doing this for years. They introduce a new variation and it takes sales away from some of their existing products. They live with it because it does more damage to the competition and they don't care if your money is in their left or right pocket. Look at how much Crest there is and see if you can find Close-up, Pepsident, Perl Drops, or Aim.
Model-T - You can have any color you want as long as it's black. By ignoring consumer sentiment Ford basically made their competition and sent their customers over.
If Nikon (or Sigma, or Tamron, or ...) produces glass to rival Canon and can make if more affordable then Canon could be lining up their customers for the competition.
To me it makes no difference, this is a hobby. I use a 50D so there is plenty of room for improvement within the current Canon lineup. I would however like a high MP camera. Not to impress anyone or to make large prints, but to put more dots on small subjects. I'd rather spend $5000 on an excellent high MP body and use "digital zoom", than to spend $12,000+ on superior telephoto lens (or maybe 2, the 200-400 and 600). Personally I think that's part of the Canon formula, they'd rather sell high $ glass than high MP bodies that let you zoom digitally.
Because Canon glass holds it's value so well, and it seemes like Nikon has less expensive glass, my next upgrade may well be to Nikon. I've got a long way to go before that happens though. I need to take care of soooo many other things before I can make any major camera purchases. Unless Canon comes out with the perfect camera before then, that's what it will take - my perfect camera, the camera people will have to wait 3 or more years to get any of my $. If they can live with that so can I.