I understand where you're coming from, but taken to an extreme it would mean we'd still be building C-141s and flying around KC-135s - oh wait
The C-17 is already fairly old technology - the engines (the F117 is really just the military version of the PW2000) date to the early 1980s and the airframe the mid 1980s.
The C-17 is a wide-body airlifter, the 141 and 135 were narrow body. It's going to be really tough to significantly improve on the C-17. Yeah, you can go to an all composite wing and more fuel efficient engines, but the result will likely be a relatively small incremental improvement over the C-17. The A400 makes use of modern airliner tech, but applied to a military airlifter the performance improvement has been incrementally small. What's driving airliner improvements is fuel efficiency. Fuel efficiency is a low priority in the military world.
As an example, take the KC-46. It's based on the 767 which is OLDER than the C-17 (first flight was in 1981, the same year the C-17 proposal was submitted.) That "old tech" airframe still works GREAT and will continue to do so for decades to come. Modifying the new high tech 787 as a tanker will be much more difficult because it is a single point design very finely tuned to a very specific mission. Along those lines, it will be interesting to see if Airbus will be able to modify the A330NEO as a tanker. To make the A330 competitive will require much more than just mounting new engines. Once the fuselage is tweaked and stretched and longer span wings added, it will be interesting to see if it makes sense to go to all the effort and expense to modify and certify it as a tanker when new-build KC-46s will still be available.