Better off? Financially? Lifespan? Mortality —All cause or breast cancer specific?
I think your question is more about prospective vs. retrospective.
Placebo controlled double blind prospective trials are the most definitive and by far the most expensive.
But they’re not always appropriate.
Suppose I have a treatment that prevents death from all causes and can even reverse death from any cause if used shortly after death.
How do I prove it works? If I keep the treatment to myself and wait until I’m 250 years old to share it, surely people will believe…but is that proof?
Suppose I give it to my family and friends and we all live to ages exceeding 200, but some die by car/boat/plane crash and the treatment can’t be applied. Is that proof?
Suppose I make the treatment fully available to anyone who wants it. And 50 years later we notice the number of humans claiming to be age 100-130 years old start to explode, but we also see plenty of people dying between 70-90. I would argue this is not proof, but it is good enough.
Suppose we put everyone in the world on this treatment and 100 years go by and the annual rate of death per drops from 750 per 100,000 to 10 per 100,000…I would argue this isn’t proof, but it is also good enough.
So how would we genuinely prove effectiveness of my treatment? Well, in a world where anyone and everyone can have it if they desire, we find volunteers or enslave/imprison people and assign some of them to the control group and others to the treatment group. Wait until 100% of one group is dead and then tell the world our results. I would argue this is proof but is evil to do to humans.