Nobody ever said that mainstream Christianity is universalist or annihilationist. The point was that you cannot state that the Christian hell is necessarily an end-in-itself, because there are theological schools which would say otherwise, and they might be correct.
The truth or falsehood of Christian theology is not a popularity contest. Universalism or Annihilationism might ultimately be true even if the entire Christian world decided otherwise, so if you're not going to take such possibilities into account, a theological argument would be more appropriate than appealing to what the mainstream says.
Well, if we are talking purely intellectual/philosophical/theological speculation, then yes: you have a point.
IF there was a supreme deity and IF that deity exhibited the characteristics most commonly associated with such entities (benevolence, wisdom, justice, etc.), then the Cosmic Auschwitz scenario can basically be dismissed.
That is not why such afterlife concepts bother me so much, though. The reason why they do has got EVERYTHING to do with how many people believe in it, and isn't touched at all by the scenario's plausibility or likelihood of existing.
See, I don't believe any of this to begin with. To me, afterlife concepts are basically wish fulfillment fantasies in the vein of
Cockaigne, with different levels of intellectual sophistication depending on your religion or particular tradition, but still plugging into the same psychological needs and fears.
What bothers me is how the world view of people who DO believe in these things affect how they interact with the rest of us and the world: their politics, their perception of people, their conception of morality, their relationship toward authoritarianism, etc.
This is where it really makes a difference whether you believe in Cosmic Auschwitz or not, as it informs your whole outlook. (Granted, there will always be individuals who manage to be kind and humanitarian even while believing the most atrocious things; but on the whole, there is a reason why Saudi Arabia is a lot more problematic as a society than, say, Sweden.)