...his loot just isn't worth it to return to this dungeon for.
That's a fair point.
The easy, and ever so slightly snarky response would be just don't go back to fight him.... I know not quite helpful.
The more complicated and useful answer is that content has to end somewhere. And while the King doesn't offer any really good loot, it would be hard to justify him dropping anything better and more worthy of the fight as quite frankly, there's nothing in the game that would be a proportional loot item.
One idea would be for the King to ALSO drop a Tome of Lost Wisdom, which grants a Skill Point. That might give a proportional reward for the fight, and give an item that would be useful, but not overly powerful at the time you can get it.
Of course, development on EK is done so it's unlikely and loot changes would ever occur but it's an idea.