[ta1243]

The shallower the angle the less energy you lose, but you are still losing altitude.

At some point you lose enough energy that your speed drops enough that your altitude starts dropping significantly. You can't lose the energy without losing altitude, and once you lose altitude you start losing energy whether you like it or not

I think what you are wondering is "can I stay in the thin atmosphere bleeding X Joules of energy for 50 minutes until most of the energy has gone rather than entering more steeply and bleeding 10X Joules for 5 minutes"

However once you lose energy, you lose your altitude, and as you lose altitude the atmosphere thickens and you start very quickly losing 5X, 10X, 20X joules every minute.