I disagree my FRiend. The B-52 has more than paid for itself and can still do things that modern systems cannot.
I can think of nothing more fearful than a B-52 dropping in anger.
By the time the B-52 reasches 2040 the plane would be 100 years old. We need new planes because we have new adversaries(the Chinese) and old ones(Russia) who investing heavily in their militaries. These new bombers would probably to be exoatmospheric and reach their targets within 5-15 minutes.
You also have to take in account the ever improving air defense networks that are getting more sophisicated every year.