I think you're missing the point. Nobody really cares how you or someone else spends his own money. At the end, it's your money, and you know better how to spend it. The point I'm trying to make is that why would someone invest into another system if Canon or third-party manufacturers have products at least as good as Nikon's or even better? Have you directly compared Nikon's 14-24 to let's say Schneider PC-TS lenses? Which exact Nikon's lens is that much better than Canon's one to justify investing into it? How much more details would D800 capture over 5diii, especially under dim light? I may be wrong but I'd think very few people make money by selling their landscape work, so only one can decide if he can justify investing into a different system.
You are missing the point that it's never the lens itself, but the lens/camera combination. I agree that if Canon had a competitive 14-24 many people would be happy with that, but still those who can would buy it along with a D800. Why? Because it's just better, and that's all that matters. A D800E captures more fine details than a 5D3 when you put a good lens in front of it. That's because of more MP, no AA filter, wider DR, less low-ISO noise, etc etc.
With lens prices going up, people want to get the max out of their investments. It's not out of stupidity that even people heavily invested in Canon glass buy a Nikon body and a few lenses too. They do so because it's more convenient. Many people with a 5D2 did so because they felt it was the best way to broaden their possibilities, since the 5D3 offers more or less the same in terms of IQ.
One very important reason why it would be very silly (being polite) for a pro to use both systems, is that if one of his/her cameras breaks down, s/he cannot use the glass of one brand on the other camera which is of another brand.
Do not forget NPS and CPS work very differently from one country to another - very different regulations, set up and benefits. They might offer a replacement while they fix the broken one, and that replacement might be free, or it might not. But regardless, by the time you get it to Canon you have missed the shot(s) you were trying to get.
For landscape photographers that may not be a problem (I do not know as I rarely shoot landscapes), but if you are at a sports event and your long camera dies leaving you with only a wide angle camera, you are going to have problems, very big problems. If, in this scenario, you have the same brand of camera you can swap and change your lenses to your hearts content if one dies. Might not be ideal to only use the backup but at least you do not miss the shots and potentially your job or rep.
Now, there may be some very rich pro that can buy every lens for every brand to cover such an eventuality, but most can't. And even if they could who is going to carry that much stuff to an event!!
I do not care if you use Nikon, Canon, Sony or a paint and brush, but using both is just asking for trouble. It makes no sense.
You must be incredibly cool to call all who shoot both systems silly.
You have made clear that you wouldn't manage to handle any camera with a different control layout, but you're making the mistake to think that it's the same for everybody. I have an iPad and an Android smartphone, and so far I didn't go crazy using both.
You're also making the mistake to think that one must have the same set of lenses on both systems, but no one said that. I have a Canon crop and Nikon FF (D600). I can assure you that I replaced the Canon 85/1.8 with the Nikon G version without any regrets - optically another world.
On top of that it costs less to buy a D600 + Nikkor 85mm than a 6D + Canon 85mm. For crop upgraders keeping their Canon crop AND adding a Nikon FF is a win-win situation.
I think I will keep shooting Canon for the long teles and I'm very interested in the upcoming crop line, but for all the rest Nikon and/or 3rd parties provide better stuff or at least a much more attractive value for money and as someone said, I'm not into feeling a hostage of a company or another.