Survival of the fittest just means the most adapted to the current environment. Our current environment has medicine so we’re adapted to that. If that suddenly changes then sure it would be an issue, but so would a climate difference of even a few degrees, a slight difference in the chemical make up of air, etc.
I feel like that’s the same as saying people are toxic or socialisation is toxic. Yes, toxicity can exist online, it can also exist in the supermarket, at school, in a field miles away from anywhere. It’s just a facet of being with other humans.