Replacing the KC-135 fleet with 15 year old airplanes such as 767s, or A310s at a fraction of the cost of new, and keeping them ten years, makes economic sense.
No it doesn't. First you need
170+ second hand aircraft to be available -
which they aren't. Then you need to bring them to the same standard. Then you need to modify them to KC configuration. And how long would it actually take to buy up 179 second hand aircraft (even if they are/were available) and convert them?
And as has been pointed out many times on this thread, you then have to replace them again in 10 years time. With what? And at what cost? You have to just start the whole process again.
Second hand aircraft may make a viable proposition to an airline or charter outfit that cannot afford new, however, it just doesn't make sense with a capability you want to last for another 20, 30 or 40 years.
Planes designed for the military are often worn out or obsolete in several thousand hours.
Well by your own examples that doesn't apply to the USAF KC135 fleet which have already lasted 40+ years, does it?
That would be because........
they were bought new, maintained well, upgraded when necessary and have a low hours utilization
in comparison to airline usage.