I was 15 when Dear Doctor first aired and had no interest in biology, I chose to do applied mathematics instead. I found the arguments that humans might not have evolved if the Neanderthals had still been around to be a convincing reason for letting all those people die.
I'm a big Discworld fan and later that year I read the Science of Discworld and it explained how evolution by natural selection works. I found the theory fascinating and found out much more about it in other publications. I realise now that not giving the cure to those people is like not using a cure for cancer or aids in order for chimpanzees to evolve into intelligence.
That's not how evolution works!

It is one of the dumbest things to ever happen on Star Trek, including The Final Frontier!
I still think that the tone of the episode works, and there's some good character stuff, but an entire civilisation will die out because Denobulan understanding of evolution is fucked up.

Hopefully they found a cure for the disease on their own, built their own starships, declared war on the Denobulans and wiped them all out. That explains why they don't exist in the 23rd century.