The 747-8 significantly changed the international network, leading to 747 trips being constructed differently than they were even in 2016/2017 with the 400. More longer legs mean the tails rotate back to ANC (or SDF) more frequently, allowing for a crew change and construction of shorter trips which are more efficient for the Company.

SDFZ operates completely differently within the international network than the Whale, and obviously the 767 doesn't have those kinds of legs to rotate crews as frequently.

If some hypothetical eventual new airframe does "one leg and done", it'll go senior because of 1. new shiny and 2. schedules and early mass displacements for training.

I hope its an A350F though, if only so the Boeing people bid it with gritted teeth