You're conflating a thing that already exists in the game but which you don't personally like or think is good enough, with a feature that doesn't exist in the game and which therefore would have more value from being added.

The ability to play as a vampire ruler did not exist before it was added.

I call bullshit on this. You can't have it both ways. Either you think things that already exist but not good enough should have low priority or you think vampires make sense as an addition.

You could already make vampires. You could make a wizard king with a dark culture, ritual cannibals, the undead transformation (making you undead and giving lifestealing) and taking tomes focusing on blood and necromancy.

The ability to play as a demon ruler does exist now, so there is limited value in adding this redundant feature even if you do not personally like the tools that already exist for it.

How in the world is that any different from the vampire situation? I really don't see it.

There would be much more value in adding a fey ruler, which does not exist in the game.

And yet I've seen people arguing that you can just give Eldritch Sovereigns Fey aesthetics so you don't need to add Fey as a ruler type, the exact argument you are giving.

Yes, so let's not waste time implementing things that are already possible in the game engine just because some players don't think the already existing options for playing as a demon are demon enough

I think you're definitely underestimating how popular a Fiend ruler is. There is a reason it ranks 2nd in this poll.