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.
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.
A Jake Brake is essentially using the engine as a compressor and then venting the air at TDP, that's why they make such a racket.