...but others have pointed out that in DnD canon lore, apparently vampire spawn go to Mephistopheles regardless, so the only thing that changes is that he gets them a little later and no one gets the powers, if the ritual isn't performed. This ambiguity I think does a big disservice to this overall story.
Well, if the spavn's end up ending up at Mephistopheles anyway, then I don't see a moral dilemma with getting the power at all... And I apparently haven't learned anything, since every pass the spavens are sent to their destination...
I didn't find anything about that. If it were so simple, Cazi could just made spawns and kill them immediately, until the 7000 are full. The fact, there is this ritual would suggest, that this isn't the case - at least not in BG3 universe. So basically you condemn those spawns to hell on top of having to kill the whole Gur tribe, which often seems to be forgotten by people. The Gur are innocents too, so not matter what, letting Astarion ascend is the evil solution. Which is ok, if you want to roleplay that, Yi just want it acknowledged.
"We are all stories in the end. Just make it a good one."