> ...but the B-2 is now 30 years old and was designed in the mid 1980s.
Given that the B-52 is 65 years old, built with 1955's technology, I would expect something more sophisticated to have a longer shelf life than 30 years.
That's not how it works. There are loads of planes much younger and more sophisticated than the B-52 which are no longer in service. To name just a few high profile examples: the SR-71, F-117, and Space Shuttle.
Given that the B-52 is 65 years old, built with 1955's technology, I would expect something more sophisticated to have a longer shelf life than 30 years.