Wear is wear, and it is cumulative. If even light wear (which I don't
believe this actually is) can be avoided, it should be avoided, but I
buy your wear theory about slowing for hills though. I used to use
cruise control (constant speed), but I have dramatically increased
fuel economy (which indicates less wear of course) by manually
driving
constant RPM (which made me slow for climbs, but speed up for
descents) instead. I wish my vehicle had an option to use either
method.

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The way I learned it that is Heat = Wear. The wear on the brakes is
obvious as this is a friction system that directly wears on the
components.
When engine braking most of the energy is used to compress (heat) the
air in the cylinders of the engine. The engine has a very good cooling
system to deal with this. Wear on the drive train should be mininmal
as in a properly designed drive train the is no metal to metal contact
as there should be a thin film of oil between all the metal/metal
contact surfaces.
Hydraulic drive trains like the Torque converter in automaic
transmissions can heat the oil which can damage tranmission parts if
it gets too hot. So I do tend to avoid much engine braking with many
automatic transmissions.

Brian.