It's because we actually have little desire to continue evolving as we have.
Natural selection is a reality in that it's the way things have happened so far. It's not an ideal, and it's not something to strive for. Many biologists like Richard Dawkins openly call for people to defy their Darwinian instincts.
Species that evolve become better suited to their surroundings. Humans have now developed the ability to change their surroundings completely, so evolution no longer serves the same purpose. It's still happening, but we don't know where it's taking us. Genetic research may allow us to take the reins and drive it ourselves.
I don't think humanity is being degraded by simply making those with various problems equal in society. It just means their issues aren't as important to us anymore. Besides, natural selection is still at play because now that such people have a level playing field, it's up to them to overcome their disabilities and procreate, and they'll self-evidently find that more difficult.
- SmartLX