I for one do not think we could replace the 757's with 767's. This is a very different plane. Buy more 767's? Maybe. We do need the lift, but not to replace the 757s. Maybe the 5% routes that the uber Guppy can't handle?

The obvious huge problem with replacing the 75's with 767's? Gate space. A 767 will not fit at a 757 gate. SFO is already out of gates, EWR is worse. A 75 can fit in the narrow body gates at ORD, but not a 767.