The SR-71 is mostly analogue. It has remarkable battlefield resilience including ability to operate when EMP and nuclear radiation are issues that would affect a lot of digital systems.
Worried about how to navigate if the GPS systems gets knocked out by an adversary? SR-71 has an analogue system, use that. Worried about how an adversary could use cyberwarfar against your avionics? SR-71 has you covered, use all of that.
And sure, you could take all the designs and hand them over to the SR-72 team at LM and say "yeah, all of that please", and they could figure it out, but you know what might be handy? A working SR-71 they could play with. And who knows, maybe the answer is to take an SR-71 in its entirety, and just upgrade a few parts to get the SR-72 performance (Mach 6) that you're after.
Worried about how to navigate if the GPS systems gets knocked out by an adversary? SR-71 has an analogue system, use that. Worried about how an adversary could use cyberwarfar against your avionics? SR-71 has you covered, use all of that.
And sure, you could take all the designs and hand them over to the SR-72 team at LM and say "yeah, all of that please", and they could figure it out, but you know what might be handy? A working SR-71 they could play with. And who knows, maybe the answer is to take an SR-71 in its entirety, and just upgrade a few parts to get the SR-72 performance (Mach 6) that you're after.