Not all didIirc structural fatigue in the valiant and b52 were due to switching to low flying - estimated to be accelerated by at least a factor of 8 in the b52 case. Flying an aircraft in a regime for which it was not designed is not helpful to fatigue ( theres a suprise ). There were programs to ameliorate thus - High Stress, Pacer Plank etc. Afaik, there's been 2 crashes attributed to structural failure.
Not sure many aircraft would have landed safely as that one did after stabiliser loss.
1964 Savage Mountain B-52 crash - Wikipedia, the free encyclopedia