An airspeed indicator works so well because it directly measures the difference between static and dynamic pressure. It uses this data to DISPLAY a measure of airspeed. Nonstandard temperature does not change the indicated airspeed used by the pilot for stall speed, takeoff speed, etc. Altitude causes changes in indicated airspeed for best rate and best angle of climb speeds, however. Best rate of climb speed and best angle of climb speed converge at the absolute ceiling.

True airspeed can be found easily by compensating for nonstandard pressure (temperature & sometimes humidity, if you really want to get picky), but true airspeed is generally only used to determine groundspeed, and also, as a flutter speed limit, as my original post points out. Something I hadn't thought about but it makes sense now that I have.

The difference between indicated airpseed and true airspeed, which is what you were reading about is not important for determining aircraft limit and performance speeds, except the aforementioned flutter speed limits that are factored into the design but usually not published.

So, despite the differences between indicated airpseed and true airspeed in nonstandard conditions, indicated airspeed is the single best and most reliable guage to use to show stall speed, best rate and angle of climb speed, gear and flap speed limits, etc., since dynamic pressure, not true airspeed alone, determines those speeds.

It would be useless to try to rely on true airspeed alone to determine the above speeds in all but standard atmospheric conditions. In fact, it's theoretically possible that the airspeed indicator could have been marked in pressure units instead of translated to speed units the way it currently is, and with proper training and annotation of performance data, it would work just as well as the current system. You could say, "My airplane indicates 200,000 Pascal at 55% power at sea level" (I just pulled that number out of the air, so to speak, since it's greater than 1 atm.).