From what I've gleaned since my visit (you can only ask so many
questions while there!) that many aircraft are used in some capacity,
keeping in mind that includes destructive and non-destructive testing
as well. Some aircraft are sold to foriegn buyers, some are used for
research purposes only guessed at, and some are just recycled after a
long sleep. There is a predetermined route the bus takes through the
base, and I don't doubt some aircraft are set up close to this route
for tourists. The tour is not free. The D 21/SR 71 drone you
mentioned is not a museum but yet the Thundercheif gets the same
treatment. One can only guess its either preservation for future
public acess (there's a D 21 and an SR at adjacent Pima which I'll be
putting pics shortly) or storage for an undetermined future, eg testing.