To add to Beeblebrox’s excellent post, remember that mistakes made in the gene pool do not go away, they potentially accelerate and become worse over time. Unlike polluting the water supply with some chemical substance which will dissipate slowly over time, if we “pollute” our own gene pool with something that seemed like a good idea at the time but turned out to be fatal in the long run, those genes will potentially exist in our gene pool indefinitely. Removing them would be a mess.