“...The B-52 is kept flying because it has an immense and flexible payload capacity and unusually low operating costs.” [Rockingham, post 25]
More critical to the continued utility is the (relative) ease with which systems can be modified and upgraded. Especially avionics. The B-52H has a great deal of interior space, making any job of rewiring and installing different “black boxes” less complicated than that for fighter aircraft. The smaller combat aircraft are so densely packed with equipment that upgrades are less feasible, hence more costly, sometimes impossible.
Effectiveness in action is more and more determined by what avionics can be installed, and how they are integrated - with other systems on the individual aircraft, with other aircraft, and with ground, space, or waterborne systems. Purely physical flight performance (speed, maneuverability, etc) means less with each passing day. To date, fighter pilots are still in denial about it.
I once toured a B-52 in Orlando in the 1980s and can attest first hand that it is roomy and sturdy. At the time, the area still had a local SAC wing that was tasked with being ready for nuclear war missions. Several avionics panels were therefore obscured by canvas curtains and guarded by MPs with sidearms.