Planned burn for ETOPS is about 350 lbs. total. (Straight from the flight plan)

One turn back and burn off to get below MLW would very cost prohibitive.

Planned icing encounter, which must be accounted for in planning, can be more than fuel burn.

The APU Ops Spec, is very expensive because the aircraft that are to be designated have to be continuously monitored and managed. That is, they have to be routinely started inflight while domestic. Record-keeping for and the management of ETOPS aircraft is the difficult part, by far. One mistake can be extremely costly even if no one is hurt or aircraft is damaged.

MAX performance much better than the NG and there only 28 or so to be used.