Wouldn’t a cargo plane with bomb bay doors cut in the fuselage have the same structural problem as a converted airliner? Somehow, you have to make up for all of the longitudinal members that have been removed. Although, skipping to the end of your post - the idea of a new subsonic tactical bomber might make sense for some roles.
Some of the advantages of the B-52 is relatively low and inexpensive maintenance compared to the modern bombers, some of which must even be stored in temperature and humidity controlled hangars.
It also consumed a heck of a lot of fuel, sometimes needing refueling as soon as it reached altitude.
So a modern design would likely have modular everything, so after a given length of time the entire aircraft would be replaced a piece at a time on a wear and tear depreciation schedule. Estimated price about $75m a plane.
Next would be significantly improved fuel efficiency, which is just normal engine evolution, nothing special.
Somehow, you have to make up for all of the longitudinal members that have been removed.
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Maybe you could load and drop bombs vertically instead of loading them horizontally,, have a whole lot of openings that don’t intrude on the structural members.. or have a mechanism that squirts them out a opening under the tail.
Don't cut. add on under