Modern airliners are designed to cruise at an optimum AoA to stay inside the low drag bucket.
As you burn fuel and get lighter, AoA gets shallow. You then
need to climb into thinner air to get back to your optimum AoA.
If ATC doesnt let you, the second best option is to reduce speed to
increase AoA back to optimum.
Or, get a 787 with variable camber..
