Calm down. Yes, I'm serious, every single one of those characters would fit better than Pelagos. Any lore gymnastics they would have to do to get those characters on board ("oh look, we can bring Ursol's soul back"), however cringy, would still be better than Pelagos. Any arguments you bring against those characters, are, to me, weaker than Pelagos' "nobodysm" and ineptitude. I could pull another 20 characters out of my ass, which would still be more fitting in my eyes, than Pelagos.
That's how much I think Pelagos is a poor choice.
And the argument about sacrificing important lore character is kinda void to me. Arbiter is an important lore position deciding fate of countless souls. Putting random nobody in this position only undermines the lore. Like Bolvar becoming Lich King, like Illidan becoming Sargeras' jailer so should reliable character become arbiter