Jaguar Pilot, would you consider amending your statement to "Thrust decreases with altitude because of reducing density, after that Pressure Height is reached where the Flat Rating no longer applies?"
Point well taken Mad (Flt) Scientist, I was trying to make the point that the worst Vmcg may well be at an increased altitude (the WORST case) for a Flat Rated engine, after which point, Vmcg (actual) does reduce, and alleviation during certification is indeed available above that Altitude, or, to be precise, Pressure Height.
Regards,
Old Smokey