The B-52 has hands with the JASSM, ALCM, LRASM. No need to get close to pack a punch. Yeah a successor would likely be a LO blended wing body design, but the idea of cheap to operate big truck is fine.
There are systems that are in bombers that Rapid Dragon does not deal with, and you need to expand the transport fleet that the Army and Navy and you also want for other missions. It's great for pounding fixed positions where you can load up and program on ground but not readjust in the air based on how it's going.
Also not all munitions work effectively with it. A dedicated bomber enables things like MOAB.
The B-21 is the B-52 successor, and has to worry less about being shot down.