I think it’s purposely misleading to say you believe the “minhag” of MO can only go back as far as YU. I get what you’re saying, Joseph, but the point is obviously that people within the MO are keeping some form of minhagim (Ashkenaz or Sphard); they aren’t all keeping one homogeneous “minhag MO” like Chassidish groups do.
If the sole basis of your earlier posts was that it’s wrong to go against the Minhagim of your father, then an Ashkenaz would be better off keeping minhag Ashkenaz and nusach at an MO shul rather than dropping his father’s minhagim all together in favor of Minhag Belz or some such thing.
ZD, I think your summation is very accurate. I don’t see how they could ever get good hard numbers on the rates of MO’s freing out since many leave to become Chareidi or move to Israel.