Well idk, the only person that could defeat Vlaakith is Orpheus, which is something the game keeps reinforcing through Voss/Lae'zel.
And I'd agree with that because Orpheus is the only one that could be used to get a sizeable amount of Gith to rally around, and by comparison, Lae'zel is a nobody and Voss, while a very well known Kith'rak, is still one of many (hundreds maybe?), and for them to say that they are the chosens of Orpheus, it's just their word vs Vlaakith in that case, no?
But also if Orpheus sacrifices himself in the end, they've lost the war vs the "Grand Design", given that he's, seemingly, the only one that can prevent the Grand Design.
(Although Larian seems to think that killing 1 Eldar Brain is enough to prevent it completely..?)
The other thing I was thinking about Orpheus is, I don't think it makes much sense that he would be willing to become a Mindflayer in the first place.
Given that he's the son of Gith, he knows he is the only one that could save his people against Vlaakith (and any future Mindflayer attempts at fulfilling the Grand Design).
(And then there is his personal thoughts on Mindflayers in the original post)
While he needs help our help in defeating the Brain, he is also the one thats preventing our conversion.
He could just easily force you to convert and there would be nothing you could do; Kill him and its Game Over, for both the world and the game.
I suppose a companion could take your place, if they had high affinity, or for story reasons like Karlach.
But I imagine it would be pretty unpopular if your were forced to sacrifice yourself/companion no matter what, so they allowed Orpheus to deus ex himself instead (even tho they could just remove the requirement instead)