[schwap]

There's nowhere near enough inertia in the rotating assembly of an engine to significantly slow down a vehicle.

[jacquesm]

It's not the inertia that does the job (that keeps things going, actually) but the compression and shedding the compressed air that will slow things down. But for a big rig doing that idling it won't be enough, especially not on a descent with 25 tons pushing you downhill.

[BenjiWiebe]

It's also the friction of everything turning. And you've still got your alternator, oil pump, water pump, fan etc that are removing energy.