The economics of buying a plane to last for 50 years is laughable.
Why?
Most of the KC-135 fleet have had two engine changes, from J-57 to TF-33 (from retired 707s) to CFM-56.
So what? It is almost certainly cheaper to re-engine than to replace the entire fleet every 10 or 15 years = better value for the taxpayer.
Nevertheless these aircraft are over 40 years old and maintenance costs are increasing, with airframe corrosion being the worst problem
And that will happen if you buy second hand 767, DC10 or MD 11s. The first 767s are now verging on 30 years old and the first MD11s are already 20 years old. - Source: Boeing
According to Wiki, the KC-135 fleet is largely used in theatre, while the KC-10A are used over the pond
As any university student will tell you, be cautious of anything on Wiki = find a primary and reliable source.
There may even be used 737s that fit the mission
But there are no 737 tankers, nor are there any 737 tanker designs. Nor has anyone, not even the Israelis (not a slight on Israel please note, just a comment on the country's ingenuity) offered a 737 design.
The USAF should contract with Fedex and UPS to take their half-life or third-life 767s and MD-11s, and convert them to tankers.
Hmm. The first potentially viable proposition. But do Fedex want to sell these aircraft? Are there enough of them to meet the USAF first tranche requirement? They are still ageing aircraft which adds to, by your own quotation above, the maintenence costs of your fleet. AND you now have 2 different types of tanker on top of the remaining KC10s and KC135s so four different tanker fleets to maintain, with the ancilliary spare parts, logistics, training and GSE increases in cost = not good value for the taxpayer.
The A330 refueler is supposedly scheduled to replace the KC-10A fleet
Nope, tranche 1 is supposed to replace the remaining KC135E models.
But nobody offered the MD11. Are there sufficient available to meet this order? I doubt it. And it still leaves the problem of buying in and paying to standardise lots of aircraft from different airlines. And then paying to convert them. And then paying to maintain them
through life. Which is likely to be more expensive to the taxpayer as you will not get 40 years out of them, or if you do you will still face the same problems as operating and maintaining 40+year old KC135s!!