From what I know, what actually killed the XB-70 was ICBM advancements.
The B-52 survived by becoming a low altitude bomber and an excellent, cheap nuclear-capable cruise missile delivery platform that was comparatively cheap to operate at the same level of effectiveness as the B1-B and B-2A for similar roles.
The B52 was never a low altitude bomber, you're thinking of the B1A, which was to be a high altitude, super sonic bomber but SAM missile tech advanced faster than the development of the bomber could happen, and it was canned, before being resurrected as a subsonic, low altitude bomber (the B1B). The XB-70 couldn't make that switch because it was a ridiculous design (for starters).
The B52 role was to have wings (and nuclear bombs) in the air constantly over the arctic to act as a deterrent. The only way that idea could attempt to survive out of the 50's is if they (or one of them) were < hour from a target at all times. They're huge, they flew slow, and their flight path didn't change much. The whole multi-decade mission was a huge waste of money; a stage drama.
The B52 also became a heavy bomber in Vietnam, with incredibly heavy loses. The Air Force in general in Vietnam was a shit show -- observe the F4.
It's only when you have air superiority can you dredge up a B52 to deliver a payload.
These are not winning strategies in a war that would start today by an adversary like China. We're going to be caught with our pants down with B52s as the stains on our underwear.
The Vietnam era is totally different. LeMay and Powers were sociopaths whose goal was to nuke things and intercept things that would nuke the US. Survivability wasn’t really a consideration - assuming you finished nuking the Soviets, there would be nothing to return to.
The needs of Vietnam required different tactics, and the airmen paid in blood for the myopic vision of the leaders.
Eh, the B52 has survived because it is amazing at being a bomb/missle/etc. dumptruck, and doesn’t have anything fancy which is expensive to maintain or particularly brittle.
Even the airframes are mostly still alive 50+ years later, which is mind blowing on aircraft. They have made a new B52 since ‘62 but they keep on trucking.
It isn’t super fast (the B1 is that), or stealthy (B2 or B21), or sexy (B1), or nimble (any of the generations of fighter bombers that have come and gone). It doesn’t fly particularly low. Or particularly high.
But god damn can it carry and drop absolutely massive quantities of whatever type of ordnance you want, pretty much anywhere you want on the globe, and do it over and over and over and over again. 70,000 lbs worth
The 737 is still around for pretty much the same reasons.
Any alternative ends up being less effective at the same tasks, or more niche/expensive.
They’re basically the alligators or horseshoe crabs of the airplane world. Or the M2 heavy machine gun. Or the Cessna 182 or Robinson R22 of the GA world.
The B-52 survived by becoming a low altitude bomber and an excellent, cheap nuclear-capable cruise missile delivery platform that was comparatively cheap to operate at the same level of effectiveness as the B1-B and B-2A for similar roles.
Russia more or less mirrored this with the Tu-95.