@kanol, yes it definitely is.
@glscarves, while demon king lives, yes... he is "immortal." But he's not invincible.
Surviving with 1 hp and who knows what kind of injuries, means you're not gonna be protecting much of anything. Like the poster said... because of that trait, he has little to no incentive to defeat the demon king. And that, with his personality, he's unlikely to actually put real effort into getting stronger. He's already been beaten twice now by a demon general and the MC. Who know what else out there is stronger than him.
The demon king on the other hand... well, sure he could help the "hero" be stronger as a shield. But knowing the "hero's" personality/behavior, there's a risk that if he is strong enough, he could try imprison the demon king instead to maintain his immortality. Not likely something a demon king would ever allow.