The A330 is still selling well and in full production. It was introduced in the 1990s.
The 757/767 joint range is 10 years older, from a previous generation, introduced in 1982. That's over 25 years ago. The 757 has of course ended production and the last few 767s are trickling out before the 787 takes over. Many of the earlier 767s have already gone for scrap. It does seem somewhat inappropriate for Boeing to have based things on such an outdated design which must be apparent to all, let alone the well-known wing vortex issues for the type which as stated above are inappropriate for a tanker with aircraft in trail, or the inability for the aircraft to multi-task as a transport in the same way as the A330. We know the 787 wouldn't be ready for adaptation for a few years, but that would be no reason for the USAF to be stuck with paying big bucks for old technology. Tankers tend to last for ages, the KC-135s have been around for over 50 years. When they were ordered in the early 1950s they were not based on technology introduced in the late 1920s !