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.