and therefore getting 230 kts indicated, if the wing doesn't know the difference between 100 ft and FL 390, why is engine any different? Why doesn't the high EAS/IAS of 230 kts mean engine is getting enough air? If engine is approaching starvation, how is wing happily thinking "I'm flying along at a lovely 230kts
You are right about the wing if you ignore Mach effects, but the engine is very different. It is a thermodynamic machine turning fuel into thrust. It is definitely getting "enough air" at high level. The big difference is the thermal cycle conditions. At ISA sea level, input static temp is +15 deg C, at FL 350 it is around -56 deg C. This means the compressor has very different inlet conditions, and if you examine the maths of the thermodynamic cycle you can show that the efficiency is improved by that lower inlet temperature.