Yep, mega-impact is a classic example of an ad-hoc hypothesis. For example, Moon formation is much better explained by multi-impact hypothesis, which also requires less assumptions.
Dang, I can't post anything in this thread without someone thinking I agree with them that science is BS. Giant impacts aren't actually surprising in an early solar system that hasn't hit steady state yet. If multiple impacts better explain all the evidence for Theia, including the weird patterns of isotopes and possible fragments deep in the mantle, that's news to me.
