One step forward, or two steps back?

Sir David Attenborough has recently articulated a theory that has been buzzing around in my mind for some time.  It's not a very nice theory, I warn you now.  It goes like this - Darwin's theory of evolution is based upon the "survival of the fittest", but the other side of this coin is the non-survival of the not-fittest.  In other words, for a species to evolve, the fittest must survive, but equally the weakest must die out.  But as far as humankind is concerned, advances in medical science mean that babies who a hundred - maybe even fifty - years ago would have died, are now surviving.  Like I say, it's not a nice thing to think about, but is this trend interfering with what otherwise would be the natural evolution of humanity? Might we stop evolving - maybe even regress to a weaker, more debilitated species?