I'm a noob so pardon my ignorance. So my understanding is that as the plane gets lighter during the flight, its mass reduces therefore reducing the lift needed to maintain the altitude. At those altitudes and speeds, my understanding is that we can't engage slats or flaps. As a result, we would have to reduce thrust to reduce the lift to account for the loss in fuel and maintain the same flight level
So what can one use to continually reduce lift coefficient at those altitudes to maintain constant airspeed and flight levels? I understand that ATC also clear aircraft to higher FL thereby reducing the air density and therefor reducing lift and enabling higher airspeed but that still leaves us with the problem of flying at constant airspeed at cruise altitudes.