I've wondered this. it's important to realize that Survival of the fittest does not mean the same thing for a civilized people as it does for other animals. We don't yet understand all the traits that contribute to our own survival and it's possible that some of those that might be weeded out by natural selection could prove useful in the future.
Will our medicine adn other practices mean that individuals who would not normally survive, grow and procreate? yes. Is that bad for the species? I'm not sure. In my experience those with the best genes tend toward others with the best genes and those with the worst tend to the worst so I think there will be a core of people that have the qualities needed for survival in tough times should civilization collapse.
My real concern is the weight of carrying all those people with bad genes causing the collapse. OTOH with medicine able to identify and fix the most obvious problems in the near future it may never be an issue at all and natural selction will progress beneath the surface, operating on the parts of the genome we leave alone or don't understand.
BTW there is recent evidence that the rate of mutation in humans has increased mostly due to the surge in population in the last 5000 years
The survival of the fittest is still in effect. However the definition of "fittest" has changed & no longer applies to strictly physical fitness. The fittest now adapt to cultural changes rather than survival of harsh environments.
No doubt advances in culture & medicine have allowed many that in the past would not have survived long enough to reproduce to do so. However, physical robustness is no longer a major survival factor. Perhaps the existence of those in need of help tends to increase empathy among our evolving culture(s). Certainly diseases like diabetes would have doomed many major contributers to our culture to a short & unproductive life in the past.
Not having survival of the fittest causes would mean no natural selection though mate choice and environment.
Though there always will be mate choice, so some natural selection is occurring. Also there is some natural selection occurring in people who have such genes making them unable to survive certain circumstances, such as genes causing mutations at birth making them unable to live.
But that obviously is significantly less natural selection then we've had before, and is affecting us. Two negative things are happening due to the lessening of natural selection: Increased mutations left to exist in world, and our species not evolving as much in a specific direction.
The increase in mutations can be helpful because it increase diversity, which will help combat diseases, it will often be curbed if it gets to far out of hand by the mate selection we still have. Our species not evolving isn't very harmful because we are surviving in our environment and have very little competition to face.
Overall the decrease in natural selection will not affect us significantly because the more harmful genes will still be eliminated through natural selection. And it will increase diversity to help us have better chances to evolve in the future during a time of pressure.
It depends on how you explain harmful. On one hand, it is, as by keeping people alive who by natural selection would have not survived we are in a sense preventing our own evolution and any further development of our species. However, the lack of survival of the fittest could be said to be beneficial to society as it gives us something that other creatures do not have, more of a conscious perhaps? I'm not sure if that helps but that would be my opinion on the matter.