The process of evolution occurs on a time scale much longer than what humans can observe--tens of thousands or up to millions of years. Note that all recorded human history is not much more than 5000 years, nowhere near long enough for any type of evolutionary development. Things like eyebrows and eyelashes probably evolved a million years ago when they DID serve a life saving purpose. By the way, this purpose does not have to be a matter of life and death itself, but *on average* if creatures with eyebrows found more mates and and had more children than creatures without eyebrows, then the eyebrow ones would be chosen by natural selection. Maybe eyebrows are there because they make us attractive to a mate? Point is: it doesn't have to be life and death. As long as the trait is something that results in more offspring, or healthier ones, then it will dominate over the millennia and will be carried forward by evolutionary processes.
As for the second question about medical science causing devolution I think you need to take a broader view. The development of the human mind and what it does is also just part of evolution. If we create medicine that gives immunity to a virus by using our superior intellect then that *counts* just like an eyebrow counts for evolutionary purposes. We are natural creatures, and even though we drive cars and invent medicine, those are things done by the human animal, and as such they will be judged by natural evolutionary processes. It may take thousands of years, but it will happen. You will not see anything changing over a few generations, but over many thousands of years it could indeed be that certain diseases will disappear, or there could be a huge plague that wipes out half of humanity because of our tinkering with disease and genetics, etc. We can't tell, and the time scale is so huge, we cannot even see it. Evolution happens so slowly, we cannot have any effect on it. We are not slowing it, or devolving. According to the theory of evolution (which may be wrong by the way) evolution is a very long term process and hence we cannot alter it as we are just animals--although smart animals. The laws of evolution affect us just like any other animal.