Etudiant, it's not that simple.
Some of the manufacturing processes and alloys used 70 years ago have been effectively banned due to EPA and OSHA regulations. Newer processes and alloys result in parts that have different mass characteristics - which affects other parts in the engine - which in turn affect other parts, and pretty soon you need to remake the entire engine. So what you end up with is an very expensive re-development of a 70 year old engine design. All the costs of a new, modern engine with the crappy performance of the original.
When we looked at the RB211-535 re-engine ~20 years ago, they were looking at a lease arrangement. The savings in fuel and engine maintenance alone would have paid the lease costs (and that was without factoring in the fact that aerial refueling fuel costs several times more than stuff uploaded on the ground. Plus, a ~30% reduction in fuel burn means a corresponding improvement in range and/or greater payload capabilities.
HH - I doubt the USAF would ever go for a replacement 'bomb truck' - the Air Force brass want their new stuff to be fancy - they just can't help themselves. If they started out with a RFP for a new bomb truck, they'd add so much crap to it before they were done that it would end up costing more than a B-2 (I'm not even remotely optimistic that the new B-21 will come in at anything less than a $Billion per copy and will be less than 10 years late).
George, you are correct that the CMF56-2 that was used to re-engine the KC-135 (and DC-8) is long OOP, but the CFM56-7 on the 737NG isn't going OOP anytime soon (and even when it does, figure it'll be in commercial service at least another 30 years). While I can see some major obstacles to using the LEAP on the P-3 (among other things, it's significantly heavier than the CFM), I don't think a re-engine with the CFM56-7 would be too much worse than the -2. Plus the -7 is already in the US inventory on the P-8. That being said, I suspect the USAF will spring for new AWACs aircraft rather than re-engine.