It's really for two primary reasons: 1. Storing tooling, especially huge aircraft-sized tooling, has real costs associated with it. There's the physical space

It's really for two primary reasons: 1. Storing tooling, especially huge aircraft-sized tooling, has real costs associated with it. There's the physical space it occupies as well as the costs associated with keeping it from degrading. Guess who industry charges for all of this? 2. Concerns about security. If an adversary manages to surreptitiously capture images or other data on the tooling, perhaps they can get closer to developing an equivalent capability. It's certainly possible the idea is also at play that by destroying the tooling, you prevent advocates of the current system from jamming up the process of acquiring new and improved systems. It would be great if the development and acquisition of new systems was always merit-based and rigorous, but it's often not. I'm not saying this is the case, but it could have been a fear of guerrilla advocacy from Lockheed or its advocates in the Government stymieing progress. For all its awesomeness, the A-12 family was heinously expensive to operate. If you had pressure from congresspersons or ill-informed generals to acquire more of these because of their incredible capability, but that meant you couldn't afford to improve your capabilities in other areas, you might not appreciate that pressure.