The airplane flies based on how much of that mush is doing work on various parts of it, which means "I stay in the air" is a complex calculation of speed and pr

The airplane flies based on how much of that mush is doing work on various parts of it, which means "I stay in the air" is a complex calculation of speed and pressure. Factoring in the slowly-decreasing density of that mush to those very important speeds saves calculation time, which is why pilots stick with indicated as the baseline. It's a known-wrong measurement with some valuable uses, and true airspeed and ground speed are usually available in the cockpit in real time. They're just not flown by.