Okay, I didn't read the whole discussion, but the last post brings up quite some thoughts, maybe it's because of my field of study, dunno.
First of all... don't forget the reserves in AMARC...
You can't keep them flying just for the sake of non-disposing them.
They are out of service life time, in most cases anyway, which means you can't continue to fly them without major overhaul. Or else you will suffer reduced reliability and even losses.
The old components/parts will suffer from problems sooner or later, and things as metal fatigue might come unexpectedly sooner than originally anticipated.
Then there is the upkeep, don't forget that modern systems are build with reduced operational and maintenance cost in mind. The older designs have more moving parts, less time between overhauls, worse Mean Time Between Failures, etc. They were build for manufacturing, and designed for their purpose, not for their economical life. Don't forget that ideas of all kinds of Quality engineering in manufacturing and designs started after most of these aircraft were first designed!
Next comes the operating costs, which are higher for older systems most of the time; less ecnomical engines, more support needed.
Then the operational viewpoint itself, maybe the most important, these aircraft are no longer compatible with current systems and weapons. You will have to do major upgrading/updating, which is very expensive. Capability wise the designs are limited in one way or another, upgrading them won't mean that they will be as good as more modern designs, and even if they are upgrading them in future again is not feasible.
Upgrading/redesigns/overhauls take time, which means even lower availability rates for aircraft fleet already suffering from low availability.
Without upgrading, pilot workload is the same as in the past, which will probably result in more losses or higher requirements on training.
These aircraft have served their lifes, and are paid off, they are sunk costs, so don't mind about keeping them when you need to do investments anyway on them, which you might as well spend on new fresh aircraft.
They can still serve a purpose, and create value, beyond the scope of what was originally imagined. They are written off, but you can still sell them on the market, sell their parts/components, recycle parts, components, materials. Or you can find other uses for them, which brings me to your question about the A-6/A-7...they are used as artificial reefs to defend your coastline!! Different kind of defence, but still valuable to the US. Another example are the F-100, F-106, F-4 and soon probably F-15s that are converted to drone and used for real-size airborne targets. And also instructional airframes for training and educational institutes.