One would assume that even being out of production some maintenance is required and outdated or defective parts have to be replaced. Of course, in the USAF that kind of maintenance and depot overhauls can be done by the Air Force ordering new parts by open bidding, if the required tech data is on hand, so the original manufacturer need not be involved. But I do not now if that depot approach applies to the Stinger.
One of the problems brought by the War on Terror has been the necessity of focusing resources on less sophisticated combat, and Al Queda et. al, usually does not have aircraft to be shot down. Back in the 1990's we were planning on using Cape Canaveral for the training area for FL National Guard Hawk missile teams so they would not have to go all the way to VAFB on the west coast, but they disbanded those units. Obviously, someone figured that there was a greatly reduced need for air defense by our ground troops.
In the late 70's we were facing problems with maintaining aircraft such as the F-106 because some of the original manufacturers had gone out of business or been bought by other firms and even the technical data had been thrown away. And the companies that owned the data that was available were not interested in making the parts any more and would even give us what data they had and let us find someone else to make the parts.
The Space Shuttle was supposed to replace ALL US space launch systems and in the early 1970's we quit even building new rocket engines for Atlas, Thor, and Delta and just used what we had left over. After the Challenger was lost in Jan 1986 we went back to the some old rockets and it turned out we could no longer buy some of the parts required to build new rocket engines.
It is said that one of the errors made by militaries is that they plan to fight the last war; that also equates to failing to plan to fight the one before that.