You can do a lot with upgrades. And, yes, it should be possible to manufacture these again if necessary. I once heard that all the factory machinery for every bomber line was mothballed—hope that is true. With the coming bankruptcy of the USA due to entitlements, I think the probability of a new bomber is about zero.
Looks like a LOT of planes:
http://deanoinamerica.wordpress.com/2012/02/09/the-boneyard-at-davis-monthan-air-force-base/
I'd suggest that we make some SR-71 based bombers for the future.
Problem is that the B-52 can fill several roles that would require several new systems to replace.
From large area carpet bombing to loitering with large, ready missile loads, it can do a lot of things that would require multiple programs, and megadollars, to replace.