The Case for the Second Body

Not redundancy — dust. Every sensor I have ruined was ruined during a lens change in the field.

Two bodies with two lenses mounted means you never open the chamber in wind, sand, snow or rain. That is the entire argument, and it is worth more than any specification difference between camera generations.

The secondary benefit is framing speed. A wide body on a strap and a long body on the beanbag lets you take the context shot and the portrait within the same ten seconds, which is often all the animal gives you.