There is a big difference in breast cancer in older women and younger women.  Many believe that breast cancer in older women is like prostate cancer in older men, it is much less aggressive and is unlikely to result in death.   I know of women who developed breast cancer in their eighties and lived with no treatment for more than ten years, dying of other causes.  Breast cancer in younger women tends to much more aggressive, also like prostate cancer in younger men.  Triple negative cancer is more common in younger women and more aggressive.  I believe the biggest problem is that different people respond to different medications.  Just as some antidepressants work for some people, and others do not respond to them at all, but do respond to other medications, it is the same thing with chemotherapy.  All the time, scientists are learning more information about genetic markers that can predict responses, but for every marker that has been identified, there are probably hundreds that are completely unknown.  I sometimes think of that early Star Trek movie where they come back to the twentieth century and Bones sees the woman waiting for dialysis and mutters "barbarians" before curing her with his handheld medical unit.  I am sure that in 100 years, people will look at our current medical treatments the same way.  But each of us is helping contribute to medical knowledge and hopefully helping ensure that women with breast cancer in the future receive better treatment than we did.