There are a couple of things that could keep the B-52 - or any old airplane - flying past it's "design life".
1. Design tools. Planes designed back in ye olden days didn't have the advantage of finite modeling, etc., that today's engineers do. For the same flight profile(s) and service life requirements, today's aircraft use thinner gages and more efficient structural designs because we have the tools to more accurately predict stress and loads distributions than in the past. In other words, analyze the design of an older plane with today's tools, and you'll find many of them have greater margins than originally calculated.
2. At one point in its life (I think it was the 'D' model), the flight profile changed from a HI
-HI-HI to a HI
-LO-MED. This caused the wing and tails to be redesigned and beefed-up for the increased loads experienced at high speed and low altitude. Now that the "standard" flight profile no longer includes low level ingress, you get an increase in fatigue life.
3. There will likely be some kind of service life enhancement program to deal with fatigue critical areas. This may range from adding doublers to re-winging the plane. Something like this would happen during a phased maintenance period, and while expensive, would be a lot cheaper than building a new bomber from scratch.