I think there would be numerous reasons buffs wouldn't operate that high... the cold, difficulty of reaching and maintaining that altitude at combat weight, instability of flight (rendering accurate bombing more difficult), etc. I agree that historical buffing usually occurred much lower than the 35K we routinely see in AH.
The Lancaster only had a ceiling of 22-24K. Can you imagine what that is going to be like when we see it?
B-29's (after air superiority was gained) firebombed Japan from low-medium alt.
Most of the medium bombers of the war had ceilings lower than 25K.
It would mean a shift in tactics (those buffs
will need an escort!) but historically you would be entirely correct.
Of course most of the warring sides (U.S. aside) sent their heavies at night... night fighting?