If you look at one up close, you'll see that the forward fuselage is full of wrinkles and the wings have seen better days. It's true that there have been many upgrades to the avionics and programs to beef up the wings, but the planes are really getting old. Those wrinkles come from the fact that the BUFF was designed as a high altitude penetration bomber (B-52 design work started in 1945 and the first flight was April 15, 1952), so the fuselage wasn't built for the stresses of the low altitude penetration missions that became normal following the U-2 shoot-down in 1960. That's one of the big factors that doomed the B-58 Hustler, too.

They've been discussing replacing those 8 old turbofan engines on the B-52H (the only model still active) with 4 newer generation engines for a long time. Those old engines are relatively fuel inefficient compared to modern engines and are getting hard to maintain.