Unless all of Joel's posed bird pictures start coming from the 5D IV I think my next purchase will be to upgrade my 1D IV to the 1Dx II.
Watching his future posts for guidance

At this point though the 5D IV seems to be too much of a compromise. I still occasionally use the 1D IV over the 5Ds R when I need weather sealing, speed or tracking. The 5D IV may gain on the weather sealing, but with the small battery I would bet it is still gimped on speed and tracking vs the 1 series bodies.