Uhh... that's not correct. Air temperature, density, and pressure decrease as you increase in altitude. Planes don't "fly around 36,089 feet" because of an existence (or lack theof) of an isotherm. "Best range" and "best endurance" airspeeds and altitudes vary based on aircraft design and gross weight, and you can't usually fly at the most optimum altitude anyhow due to Air Traffic Control (ATC).