Springer - I agree with you - -and used the same approach to the question- take it to it's illogical extreme.
The puzzle tho - is what is the relationship.
What if-----
we put the engine air intake into a magic box - which held a constant STP as we acsended -
The engine would -of course - pick up speed as we went up . (load decreases)
but how much per thousand ft?What kind of an rpm increase curve would we get?
again -what if------ we let the engine breath normally the air as it thinned but put the "fan load " in a magic box -which retained the same air density .
the engine would again loose rpm -- but what would this loss curve look like.
I guess this all sounds silly to those with aero engineering degrees - but I really would like to know the answers .
The why is:
I get plenty of flak from "experts" who doubt my rpm readings on given engine setups - because I run the engines at 4500 ft elevation.
The claim is that I get higher readings because the air is thinner - plus -the use of a tuned exhaust is of greater advantage at higher altitude.
Frankly, I think my readings would be about the same -if I hauled the whole test setup to sea level.

Further - I think my readings would increase.
Maybe I should stick to easier questions ---