[semi-extrinsic]

FWIW, the Boeing E3 Sentry (the "AWACS plane") was based on the Boeing 707.

But for a B-52 replacement, I think civilian planes have quite a few problems. For one, the wings are attached in the wrong place; on the B-52 they're more to the front, since the bomb bay has to be at the centre of balance. (Otherwise the plane becomes difficult to control after dropping the payload.) Another issue is that a commercial jet has a unneccesarily wide body, increasing drag. (Bombs are much denser than people.) A third is that commercial jets probably can't take the same level of structural loads in case of evasive action, particularly at lower altitudes. Compare the AWACS example: that airplane should never come close to enemy fire. A bomber most certainly would have to.

All those are engineering challenges, and could be solved. I guess the biggest reason is that it would cost more than you would gain.

[jokamoto]

The B-52 is already something of a case study as far as the changes required to allow a high-altitude aircraft to endure prolonged low altitude missions. SAC's switch to low-altitude attack profiles during the Cold War resulted in a huge number of modifications and overhauls to SAC B-52s specifically to allow for prolonged low-altitude flight.