A car will tend to gain speed on a steep descent. It is undesirable and illegal to drive down the mountain at 200mph, so you must reduce your speed. Mechanical

A car will tend to gain speed on a steep descent. It is undesirable and illegal to drive down the mountain at 200mph, so you must reduce your speed. Mechanical brakes do this by converting that energy to heat and brake dust. Regen brakes do that by converting around 70% of that energy into battery storage to be used later. Your sack likely does not roll down the hill with zero losses to friction along the way. The sack has the same energy as it did before you moved it, but it does not mean the journey consumed net zero energy.