I'm surprised at the apparent difficulty in (re)designing a modern boom, especially when legacy hardware has been doing an adequate job for 60+ years.
That's exactly what Boeing thought - at first. That's why they stuck to the KC-135 boom - at first. But when Airbus's proposal included a significantly better boom (much larger safe operating envelope) and the KC-135 could not match it, Boeing went to the KC-10 boom, which was equal to the Airbus boom. The "problem" with that boom was that it was invented in Long Beach by Douglas, and not in Seattle by Boeing. Internal politics in a big company like Boeing can be fierce. But eventually "One Boeing" (a constant mantra being pushed by Corporate HQ in Chicago) won out. As far as integrating the KC-10 boom into the KC-46, that was not too difficult, although it was done rather last minute. And that is NOT what is driving the delays. The delays are being driven by all the other "stuff" USAF wants in the airplane ("stuff" that requires an additional 50 miles of wires and 15.8 million lines of software code). The new wiring got designed and installed wrong and needed to be redesigned and reinstalled.