Go and shoot some wildlife in Africa, or some tigers in the national parks of India. It's amazing how close the wildlife gets.
Yes, in trammel-operated parks.
Sure, there are times when even 800mm on a crop sensor isn't enough. But there are other times when even 200mm on full-frame is too long, and you desperately need to reach for a backup camera with a 70-200 mounted. Not everywhere is open savannah, and not all animal spottings occur at long range. There is a lot of scrubland where a lion or leopard could be hiding ten metres away and you wouldn't know it. You can be shooting warthogs or baboons with a long lens from 50m away when a previously-unseen elephant crashes through the bushes not even 20m in front of you - and it happens often. Tracking tigers in India, you'd almost never need more than 560mm, even for close-up shots of the head; quite often, they're close enough that you need 200mm for a body shot.
This is why I do what I do.
1) Cotton Carrier. Nikon D810 holstered at the hip. 125mm macro affixed.
2) Small, light, innocuous lenses ... (20 + 50 + extender) on a belt pouch.
3) Tripod + D500 + 300mm VR II slung over shoulder.
This means, I am always set at 450mm and 125 mm as I hike. The D810 + lens holstered at my hip is nothing, weight-wise.
The 300 + D500 is on a tripod, managed easily over my shoulder.
With those two lenses alone, and a 2x extender in a belt pounch, I have a 125mm, 190mm, 300mm, 450 mm, 600mm, and 900mm rotatable option
within 1 minute's time, hands-free.
If I switch lenses between cameras, my D810 has the 300mm and my D500 makes the macro ~190mm.
If I add the 2x extender to the D500 + 300 I now have 900.
If I switch with the D810 it is 600.
Again, my tripod just stands there carrying the weight, if I need to switch.
All I have to do pop the back off, reach in a pouch, double the length (or swap cameras/lenses).
If I need a landscape, hands off the tripod, switch my D810 with a small lens in my pouch, I got it in under a minute.
The Cotton Carrier is probably the best thing to come out for a hiking photographer IMO.
The 1Dx2 is only just out and barely even available. Not sure how it performs. Sure, the D5 beats the crap out of the 1Dx in an AF torture-test (tested this the other day by trying to track a drone flying erratically, against a busy background of buildings and trees, dropping 'chaff' to try to confuse the AF - one of the hardest test subjects you'll ever find). But that's a brand-new camera against a four-year-old model. There'd be something wrong if it didn't kick the crap out of it.
The D5 AF also kicks the crap out of the new 1Dx2 as well

But torture tests against elusive targets deliberately trying to confuse the AF are quite different from actual animals just going about their business without actively trying to give photographers a hard time. I've never had any problems tracking wildlife with the 1Dx either - or the D810, 5Ds, D4 or 5D3, for that matter. If you can't track a pouncing lion with any of those cameras, the problem isn't with the camera.
With larger animals, true.
But have you ever tried to track a pepsis wasp hunting on the ground?
I am sure erratic bird flight and soccer games are problematic also.
As for the sensor, we're talking about wildlife and action here. In other words, ISO 400-6400 or thereabouts. Performance at base ISO is irrelevant - there's no point taking a high-DR photo of a blur. Performance at ISO 51200 is also irrelevant - you can't shoot something you can't see, unless you routinely shoot wildlife while wearing night vision goggles. Within the relevant ISO range, the SNR measurements are close enough to be indistinguishable.
I agree with you.
But the D500 is better than the 7DII, in every conceivable way, and it has better AF than the 1Dx (1 or 2), better dynamic range up to 400, and comparable dynamic range up to 2000, with better tonal range all throughout.
Besides, the 1Dx2 uses Canon's new on-sensor ADC and should have much better base-ISO performance than the 1Dx (although the test results are still pending) while the D5 has been tested to be significantly worse than the D4s at base ISO. It's also no better than the A7r2's sensor at high ISO. I'd expect the 1Dx2 to have a better sensor when the test results come out. Not that base ISO performance or super-high ISO performance are relevant in wildlife photography.
You just finished saying Base ISO is meaningless, did you not?
What matters in moving wildlife, I agree, is ISO 640 - 2000.
However, on a clear day, with non-moving subjects, I can shoot Base ISO 64 on my D810 ... which, I am pretty sure, trumps them all

Bears are shy creatures who tend to stay away from humans (polar bears in Churchill being the exception) or be a safe, long distance away (bear viewing in Alaska, or polar bears from a boat or Zodiac in Svalbard). You need long lenses to shoot them. If you're not shooting with a 500mm-or-longer lens, you're probably a bit too close for comfort.
Agree.
....
....
This is why even the Canon shooter still shoots primarily with his 600mm.
Been there, done that. It's also easier if you have a vehicle or an elephant to carry them.
That's a lot of lens switching which you don't necessarily have the time for, and probably don't want to be doing in a dusty or rainy environment, or when balancing on a steep, slippery jungle trail.
If they released a 300mm with inbuilt 1.4/1.7/2.0x TCs in a revolver-type device (like a more extreme version of the 200-400's TC) it would be a lot more interesting.
And that's why you don't want to have to keep switching them if you don't have to. Hence, a zoom.
Yes, if you're traversing in a vehicle, then carrying different lenses is less of a hassle.
But if you're on foot, hiking, no way are you carrying 2 telephotos.
I explained what I do, above: 2 lenses/cameras immediately ready, that span a 125 to 900mm range in less than a minute's time, unencumbered.
I will never use a zoom again. I don't like the quality on the long-end, which is the only end that matters.
Although, when shooting wildlife, you're probably using a monopod rather than a tripod.
I got rid of my monopod. FAR prefer a tripod.
The truth is, I can use a tripod
like a monopod, without opening the legs, but when I really need that stability ... no monopod can give you the absolute clarity of a tripod + remote switch, especially for macro.
Or you can do what I do, and use two cameras and two lenses. At the moment, it's a 1Dx with 200-400L attached, and 5Ds (borrowed) with 800L attached on the other (when focal length limited and shooting wildlife at ISO 400-6400, I find that the extra pixel density of the 5Ds outweighs the better low-ISO performance of the D810 and Nikon 600/800 combination). No need to remove and reattach bodies and teleconverters - and, when in a vehicle or a blind, both are within arm's reach and often firmly clamped to a support, being ready to shoot in any direction instantly.
I do ... and it sounds like we're both basically thinking/doing the same thing, with different systems, in different contexts.
I disagree, though, on the base ISO performance.
(Again, it is quite easy to shoot at base ISO on still subjects during the day ... and nothing can touch the D810 in this manner.)
The difference between you and me is I do a lot of macro work, and hike, while you travel shooting mostly on the long-end from a vehicle.
So, if you're in a vehicle shooting long distance (or in a blind), sure, two long lenses are possible to carry.
Still, in that case, the Nikon 400mm on a D5 would trump anything the 200-400 could do on the 1Dx, while the 800 on a D810 would as well IMO.
Naturally, I try not to walk more than a few kilometres with it. But, when shooting wildlife, I generally don't have to - that's why we invented vehicles and ride on mounts. Weekslong hiking trips to shoot landscapes are different.
Again, within the context of shooting from a vehicle, I think you have a great setup, where two long lenses are possible.
But, for the way I do things ... hiking and actually trying to be a
silent part of nature, peacefully-exploring (and not trampling over) the terrain, my solution works for me

Jack