So, maybe nobody has an answer for this, but on the off chance someone read something somewhere, let's open a life question.
When it comes to undeath, there seems to be varying degrees of how decay works, and what it does to a reanimated being. It seems established undeath and necromatic energy is as much an established power as shadow, fire and light.
So when we see a zombie or a shambling horror walking around, we can assume it only has enough necro magic in it to animate it. These kinds of undead are falling it bits.
With forsaken and similar, it seems there was more energy put into their corpse to help them retain more freedom of movement and thought, or maybe it's because they were more fresh when brought back? But either way they stand more independent and free to act on their own free will, provided their not controlled by a powerful necromancer.
But even then, forsaken have mentioned themselves as rotting away, perhaps not at the speed a lesser undead would, but still it's been stated it's a matter of time before anyone forsaken will go that way, suggesting the energy will wane in time, or it won't be able to keep the body held together.
But that then comes to death Knights, can we assume they have been given so much necromatic energy, they will not decay at all, or to no more a degree then to that of a regular aging mortal? It seems from the moment a death Knight is created, the dk retains that permanent visage from when they were raised, and there's never been anything to suggest they rot like other undead. They also tap into raw necromatic energy and use it in combat, absorbing life essence from the living, giving the idea they might well be immortal?