First, before you decide to switch, bear in mind that a 7Dmk3 is due to be announced within the next couple of months and should be on store shelves just another month or so after that. It will undoubtedly have the highest pixel density of any Canon camera and it will most likely have no AA filter, a faster burst rate, a deeper buffer, and possibly a better AF system than the 1DXmk2. For that reason, I would strongly advise not buying a 1DX2 just yet; it's a lot of money for something which will (probably) be 'beaten' for action by a much cheaper model in just a few months' time.
Second, the key is there is no global answer as to which lens and body combination will net you the most resolution balanced vs image quality. Fact is that some lenses hold up well with high-density sensors, and others do not. Similarly, the image quality of the sensor will vary wildly depending on the shooting conditions and subject matter; the 7D2, for example, can beat the 1D4 in all quality regards, if it's getting enough light for the ISO to be kept low. But of course if you're getting less light in there and you need to shoot at something like ISO 1600, then it becomes a very different story. Then you're talking about balancing the lens' ability to resolve for high-density vs detail reduced by noise...
As the most general rule, I'd say moving from the 1D4 is always going to be a trade. What you lose in 'reach'/density you'll gain in colour accuracy and less intrusive noise. The 400mm f/2.8 is optically clean enough that it holds up with high-density sensors (it's dead equal with the 300mm f/2.8, which holds up just fine adapted to 100mp medium format sensors), but if you ever use any other lenses, it may become a consideration.
If you want a 35mm sensor with the option to crop in just like the 1D4, look at the 5DS R. You can throw away half of its resolution (if you're often cropping more than that, I'd suggest you need a longer lens than 400mm!) and you still have a larger file than the 1D4 produces, with the same level of noise, slightly more tone range, and more accurate colour. The 5D4 is also another worthwhile option as it's that bit faster than the 5DS R and is an even middle ground between the 1DX2 and the 5DS R in terms of resolution. Compared to the 1D4 the image quality of the 5D4 should be equal when cropping, and much better when not cropping.
The 7D3 isn't out yet, but for sake of argument, even if it uses the existing Canon APS-C 24mp sensor with the processor of the 1DX2 (which is normal for the 7-series), that still matches the 1D4 in colour and tone reproduction, and swaps a small increase in noise for a big jump in resolution, which should even out, depending on how you finally view the image. If the (strong) rumours are true that the 7D3 will also get rid of the AA filter and have a fresh 26mp+ sensor, that should mean it'll beat the 1D4 in all image quality perimeters, and while some lenses could have trouble with such as high-density sensor, I have no reason to doubt the 400mm f/2.8 would be just fine.
So, to sum up: forget the 1DX2, because that's a camera made for durability more than anything else, and if image quality-vs-resolving power-vs-optical reach is what you're hung up on, the 1DX2 is actually at the bottom of the pack. The 5DS R and 5D4 will give you better image quality if you really want a 35mm sensor, or wait for the 7D3 if you want a best-available action camera.
(If you can't wait for the 7D3 and the 5DS R feels too slow for you, then you could switch to Nikon where the D500 and D850 are currently the action cameras. You'd lose quite a lot of money making that switch, though.)
