Cold War aviation is even more fascinating if you know a little about it.
Kelly Johnson, while developing the U-2, made it known that it was a 100% certainty that it would be shot down. It was not stealthy nor was it fast and there were ample examples of missiles capable of reaching it at altitude. It was common for U-2s flying over the Soviet Union to have a half-dozen missiles fired at them every flight, many of which show up on their footage. I mean, it was awful. Powers' last flight wasn't even particularly hairy for an overflight:
The U-2 flight was expected, and all units of the Soviet Air Defence Forces in the Central Asia, Kazakhstan, Siberia, Ural, and later in the U.S.S.R. European Region and Extreme North, were placed on red alert. Soon after the plane was detected, Lieutenant General of the Air Force Yevgeniy Savitskiy ordered the air-unit commanders "to attack the violator by all alert flights located in the area of foreign plane's course, and to ram if necessary".
They fired thirteen SAMs at Powers, the first of which took him down, but one of which took down a MiG-19 pursuing him.
Eisenhower, for his part, was not a fan of the U-2. It's been suggested that the American space program was allowed to lapse a little so that the Soviets could establish the principle of satellite overflight with Sputnik. That pretty much puts the division over spy planes in their true light: On the one side, you have Eisenhower, Edwin Land and James Killian, who knew you couldn't knock a satellite down without starting a war. On the other side, you have Richard Bissell, who knew you couldn't get film back from a satellite nearly as quickly as you could from a plane, and knew that satellite orbits were predictable.
FWIW, the A-12 had been in development for three years by the time Powers was shot down;It was never anything other than a stopgap but even the CIA was aghast by how not secret it was. Bissell hid that from Eisenhower. When Powers got shot down, Eisenhower relented and authorized the money for the A-12 because Corona wasn't all that great at the time. You're correct when you say that the US ceased all manned overflights of the USSR after Powers, but they still flew over Cuba, South Asia and China with stunning regularity. No less than seven Taiwanese U-2s were shot down over China.
The A-12, by the way, was taken out not by Powers but by the Bay of Pigs invasion. Curtis LeMay was super-duper pissed off that the CIA had an air force at all considering it was staffed entirely by USAF pilots that had been sheep-dipped into civilian roles - effectively, the CIA got to boss air force pilots around and there was nothing the USAF chain of command could do about it. After Bay of Pigs LeMay got to elbow Bissell aside and reclaim all CIA aircraft as Air Force, which allowed him to use the A-12 (now rebuilt with a crew of two and rebranded the SR-71) as proof positive of what a wonderful idea the B-70 would be because there were few things LeMay had a hard on for quite like the B-70. Bissells' replacement, Joe Charyk, was all about spy satellites and kissed the A-12 and U-2 programs goodbye with nary a complaint.
What's really dumb about the whole tedious affair is that Powers' overflight was all about finding targets for Jupiter missiles, which were an order of magnitude more accurate than the Thor but also big, stupid and short range. Why were they short range? Because the Army wanted short-range missiles so they could play theater operations in Europe. Which meant that we had to put nuclear missiles not in England but in France, but de Gaulle told us to fuck off, so we had to put them in Italy and Turkey, but because the range was so short we needed to make sure we'd actually hit something, so we overflew the Soviet Union with a U-2 and it got shot down. Then we put the missiles in Turkey and the Soviets responded by putting missiles in Cuba and then we got in this big kerfuffle and as someguyfromcanada points out, another U-2 got shot down and the world went to the brink of war and things didn't get better until we agreed to get the fucking missiles out of Turkey in the first place. So in the end, 1960-1963 was all about the brink of WWIII not because anybody wanted it but because of bullshit interdisciplinary politics.
The D-21, FWIW, came from the air force's perpetual hard-on for parasite aircraft starting with the ugly-as-hell XF-85, continuing through the Kingfish and Super Hustler and terminating, probably, in the ill-fated D-21. Lockheed was never a fan of it because the SR-71/A-12 was hella harder to fly than most people realize; the primary difference in "flyability" between the A-12 and the SR-71 is the SR-71 had interlocks that would kill the other engine when one of them flamed out so that the things wouldn't hammer rapidly back and forth at Mach 3 (talkin' 3-4 times a second). SR-71s would just lurch and fart. The D-21 probably killed forever the idea that any parasite aircraft should sit on top; at least the XF-85 kinda-sorta worked. And hey, so far, so good.
Worthy of note: approximately 85% of Scaled Composites' work is classified.
I'm just going to leave this here.