Don't understand what the 787 has to do with it, but for the tanker mission, the underwing weight of the GEnx-2B (compared to the CF6) is up well
over a ton (actually closer to two tons). That would have pushed the empty weight of the 767 tanker up by close to 4 tons. Unless there was a corresponding increase in the max takeoff weights (which is its own can of worms), that would mean less fuel upload. For the 'standard' tanker mission, the reduced fuel burn of the new engine wasn't enough to offset the reduced upload, resulting in a net reduction in the tanker offload capability (based on the USAF provided mission profiles). Lots of expense for no extra capability
Remember, the 767 MTOW at EIS was 320,000 lbs. - over 40 tons less than today's airplane, and the engine options were in the 48k-50k thrust range.
If there was a suitable new 50k engine available it might make a viable 757 replacement.