This is actually one of the things I love about the 100-400L. Trombone zoom is logical xD
As a side point, what on earth do you use the sigma 70-200 for?
'Definitely want to stick with a crop camera seems stupid' so many people seem to say this about wildlife, Motorsport and such, but how many of the pros do you see shooting with 7Ds and D7100's... They seem to much prefer the truly pro bodies of something like the 1DX (partly I suspect due to the AF system). And if we are talking zoom lenses to 400, the 200-400 is stupidly priced (only pros would buy them) but is arguably the best lens out there.
It amuses me however how much this thread has descended into Canon vs. Nikon. Ultimately, as others have said, it's not the gear that matters, it's the learning and the enjoyment. Personally I think the switch you're making is crazy, but if you feel it'll better suit your workflow and style of shooting then fair enough. Whilst I'm frequently looking at new gear, I'd like to think I'm more focussed on the art itself. (E.g. Predicting the shot before its really occurred)
kd
You would be surprised how many pro wildlife togs use a 7d, D300 or D7100 and how many Nikon wildlife shooters are screaming for Nikon to release a D400. Not least for emergency extra reach.
Ultimately you need reach in wildlife (as in real wildlife, not pets or ducks at your local pond, but wild bears, elk, moose, wolves, coyotes, beavers, otters, kingfishers, golden eagles). There is only so close to a bear that I want to get!
If you are lucky you can get close enough, but there are usually legal, safety or practical issues preventing you. And even when you can get close, almost touching distance, it is surprising how much reach you need. Last week I was photographing a local osprey and could touch the tree it was perched on, having 640mm equivalent focal length was only just sufficient.
Now a full professional will undoubtedly try to use a FF camera whenever possible (better noise, better sharpness). The difference is they can afford a 600mm + lens that can take extensions. They also have the time to be always out there registering hundreds and hundreds of hours of field time during which they may get lucky with closer encounters.
Then is just comes down to practicality. There are many ways to get an affordable 400mm f5.6 lens on a crop camera giving 600mm or more reach. On a FF camera there is a big wall in your way, a 500mm f/4 is bloody expensive and no amateur will wish to pay that sort of money.
Of course you can crop but if you crop say the 22Mp 5d3 image to a D crop you are down to something like 9MP, or one could use a 7D and have 18Mp covering the Same region. The7D image will definitely look better printed on an 8x10" on your living room wall.
Now I hope to buy a d800 (mortgage deposit aside...) but that only interests me in that at least I will get a 15Mp DX crop- however a D7100 would be far cheaper and give me more pixels over the same area for the same lens and the money could've spent buying a longer lens to boot.
If you can afford a 1dX and 600mm then that is a different bll park to play in!