I understand that an aircraft boneyard is an amazing tourist attraction for aviation enthusiasts.
However, the number of planes that the boneyard in Tucson has accumulated today is mind-boggling. I know that, initially, the purpose of the boneyard was to salvage materials from old and obsolete aircraft. However, the pace at which most aircraft are being scrapped is very low. Though it serves as a spectacular sight, why is the boneyard still active?
If the number of planes waiting on getting gutted isn't decreasing even though scrapping activities are happening that means there is a steady influx of end-of-life planes arriving to get scrapped.
As long as there are planes and new planes being built (most have a lifespan of a few decades) there will be planes that need to get scrapped.
You can't just dump the plane on a landfill or you will get nature activists on your case about the hazardous materials (hydraulic fluid, remnant fuel, ...) and recycle potential. Plus most components (instruments, engines, the espresso machine in the galley, etc) will still be usable and airliners will want to buy them as spares.
As ratchet freak noted, the boneyard is still active because planes are still being scrapped. One reason specific to the AMARC in Tuscon is the B-52 aircraft still being scrapped for arms reduction treaties. Planes may also be taken for use as targets, training aids, or even to be returned to service. Even if it just seems like the planes are sitting there idle, the boneyard also serves as a huge collection of available spare parts that is continuously being used to supply the aircraft that are still in service.
Typically controlling over 4,200 aircraft as well as many other types of military equipment, AMARG works very hard in promoting itself as not just a 'Boneyard' and takes every opportunity in explaining how it operates its cost effective, tax saving operations. Many of the stored aircraft can be returned to an operational status in a short period of time and there is a continual process of anti-corrosion and re-preservation work which keeps the aircraft in a stable condition during their stay.
So the purpose is not just to scrap planes, but also for storage and reuse. It's not even necessarily just for old planes; it's been used to store brand new planes until a use is found for them.
To tack on to @fooot's answer as far as aircraft reuse is concerned, AMARG also prepares aircraft for potential foreign military sales. From an FAA circular:
The U.S. military normally retires aircraft to Davis-Monthan Air Force Base in Tucson, Arizona to be stored indefinitely, cannibalized, scrapped, or restored to service at a later date. Surplus U.S. military aircraft are generally not sold directly to private U.S. operators without congressional approval. However, U.S. military aircraft may be exported via foreign military sales programs or other agreements. (emphasis mine)
This C-130 lived there for three years prior to being sold to Botswana.
Interestingly enough, many aircraft stored there are actually in better and more modern condition than the state in which the US Air Force flew them. I knew a civilian assigned there who was the back seat test pilot for F-4s after they received the necessary avionics upgrades in order to be attractive to foreign governments.
