Static discharge - whilst I might agree that the A330K would have a lot of transferable fuel, it would be utter overkill for our conceivable needs. What we do need is the right number of hoses in the right place - not just fewer enormous tankers which have lots of fuel but are thinly spread about the area!
All independent analysis concluded that a 767/A310/VC10 sized aircraft carrying as much fuel as the design would allow was the best solution. Things like 747s and A330/340 are physically too large and expensive to permit flexible basing and AAR towline manning.....