There seems to be a lot of misunderstanding as to what the KC-46 is. It is NOT a modified 767 freighter or even a warmed over KC-767. It is a very different airplane in many ways. The FIRST thing that must be done is the basic commercial configuration, the 767-2C, must be FAA certified. Only after that is done can the military stuff be added and the military stuff certified.
As for "past" Boeing tankers and the KC-46, yeah, there's a lot of handwringing inside Boeing on that subject. The KC-767 had a modified and "upgraded" KC-135 boom. So did the early KC-46 proposals. But Airbus's boom was significantly better. Attempts to further improve the KC-135 boom to match Airbus' boom failed and so Boeing switched to a modified version of the KC-10 Advanced Refueling Boom. In other words, a "Not Invented Here" (in Seattle) Douglas(!) product. Lots of political consternation and last minute engineering.
And what's with this "Frankentanker" business? Do you Airbus folks realize the A330MRTT was kludged together from stuff from the A300, A310, A320, and A330? At least Boeing used (mostly) 767 stuff to build their "Frankentanker" and not bits from four completely different airliners.