I took a look at the B-52's Wikipedia article. Apparently, it's expected to serve until the 2040s. If the last B-52 is active until 2045, B-52s will have been around for 90 (!) years. Isn't that utterly ridiculous? Imagine if any militaries were still using bombers from 1924 in 2014.
That was the development time period. Once a military problem is solved, it often doesn't need changing for a surprising amount if time. For inatance, imagine if we were still using a gun from 1911? (.45 ACP) But a gun from three decades before that would be laughably archaic.
If we could have slapped electronic countermeasures and upgradable radars, etc on them I don't really see why we couldn't have flown the old WWII style flying fortresses today. I mean the job essentially hasn't changed since then, and even back then they would still need fighter escorts.
Well, the B-52 has in fact reverted to limited role, I gather for delivering nukes it's limited to delivering stand off missiles outside the range of SAMs etc.
The fly really high concept, going back to e.g. the WWII B-25 Superfortress, ended, and the B-1 went in the direction of low level penetration (below radar, and hard to pick up from ground clutter by fighters above it, something the B-52 tried for a while, but it wasn't designed for that), and the B-2 back to flying (fairly) high, but essentially invisible to radar (it can be detected in a general sense by certain types of radar, but not such that you can shoot it down).
We had to transition from piston engines if for no other reason than reliability and maintenance, plus, commonality and safety in fuel, etc. etc. The B-37 was our last in the strategic bomber role, also see the piston powered CAS A-1 (https://en.wikipedia.org/wiki/Douglas_A-1_Skyraider), heavily used in Vietnam, which the A-10 replaced.
