all of which is irrevelent if you were to produce a product with a performance step up comparable to the SR 71when it was introduced....
Well, given that a halfway decent SAM system these days can take out anything from a Cessna to a satellite (such as the one sported by a lot of USN vessels), it'd have to be an almighty big step up in performance over an SR71. A "Rocket Ship" wouldn't really be good enough to be guaranteed invulnerability through performance alone.
Even if the A12/SR71 design was completely stealthed-up (its air frame had a pretty good RCS for the day), it's a struggle to conceal the shock diamonds in the supersonic exhaust (they're nice radar corner reflectors) and the IR heat signature is pretty un-stealthy too, no matter what fuel additives are used. This was all realised back in the 1960s, which is why
AFAIK the first few A12s had RAM wedges in the wing/chine edges and the later ones and SR71s didn't, the irony being that one of the things that held up the initial contract discussions between CIA and Lockheed was air frame RCS; they needn't have bothered. All these things were factors in the closure of the SR71 operation, and had it continued operating it would have served as a stimulus for SAM system developments even more capable of knocking one out of the sky than an SA-5.
The SR71 is probably close to the limit of what one can achieve in terms of sustained airborne air breathing flight. If you want to go much faster you run out of materials that will work throughout a sustained flight. If you want to go higher there's no air to make wings work thus requiring fuel to maintain altitude and manoeuvre, limiting flight time, and you have to take your own oxidiser with you too.
The altitudes between 100,000ft and space are called the "ignorosphere" for a reason - we've never really been able to operate there in any meaningful way other than brief hops or excursions to/from orbit.
However, an updated SR71 would be a real crowd-pleaser at airshows, and personally speaking I think that alone would justify it
