A 15% improvement in survival is a statistical number; it doesn't relate to what really happens. It may be that 85 of the patients didn't respond and 15 responded beautifully and are living disease-free for many years; in fact, that is the rule.
The use of 15% improvement in median or average survival is meaningless in human terms. What we need to do is identify the 15 patients who respond and go on to live healthy lives and not treat the other 85. That's the real problem. It's not the 15% improvement in average survival.