saddle parts fail by fatigue. IME bodyweight is only part of the equation; some folk ride hard tyres on bumpy roads, and some have a forceful pedalling style that causes the saddle rails to flex with every pedal stroke. Both these things can create more fatigue damage in a saddle's metal parts.
That said, the quality of the materials, design and manufacture of the parts makes a difference too. Spa Nidd noses are known to be vulnerable to failure, because they are made in such a way as they have an almost perfect crack-starter in the corner. In the case of Brooks nosebolts they seem less likely to last than they used to; I suspect that the material quality is variable/poorer than it used to be.
Steel is funny old stuff; it is usually specified (on an engineering dwg) to meet some minimum quality level. However depending on the way it is made and who made it, it will exceed these minimum values and it may also have some other properties that do not appear in the quoted specification. Sometimes these are tested for, other times they are taken for granted or simply not known in detail. Note that if the steel is only fit for purpose because of its secondary (unrecorded) properties, it will end up costing more if it is bought with these properties assured, one way or another.
So what happens down the line with a mature product is that some buyer decides that they can get "the same steel" from someplace else, or steelmaking practice changes slightly (in a way that is not captured in the specifications and tests) and the result is that the same parts no longer do the job properly/reliably.
We may be seeing that with parts like Brooks nosebolts; they may have been outsourced from a different manufacturer, and/or made from steel that has a different source/manufacturing route.
I have seen problems of this sort appear in many well-established products. In the case of a Brooks nosebolt, I can see it happening very easily; the bolt is fatigue loaded in bending (something which is unusual for a bolt and may not be tested for normally) and all kinds of things can alter the performance of such a bolt, for example residual stresses from the manufacturing process, the inclusion population of the steel, etc.
Is a GB saddle a better bet? Probably. Good to compare the warranties, spare parts availability and manufacturer support on offer. Of course nothing will quite make up for a saddle that fails on you and spoils your ride though.
cheers