The A-12 was built in an era when the aerospace industry was flush with knowledge and active R&D into mach 3+ flight - everyone was convinced that it was the future. Today's industry is heavily optimized for subsonic airliners and fighter jets that have top speeds of "only" mach 1.5-2. No one in the west has built a mach 3 aircraft since the 60s and not in Russia since the early 80s. The knowledge isn't _lost_ (well, we hope) but there is a lot that will need to be relearned, and we all know how even the best documentation is usually missing a lot of details.