Older missiles can be selected for tests because they would otherwise need to be destroyed. Reconditioning is expensive, and the condition of some rockets makes it unsuitable. Using such rounds for tests avoids consuming missiles that can remain in service. Trident's exact shelf life is closely guarded, so the general explanation does not establish the age or condition of the missile involved in a particular failed launch.
Notable Point · 17:21-18:27
Why aging missiles can be selected for test launches
Missiles approaching disposal can provide test rounds when reconditioning would be unsuitable or too expensive.