I wouldn't worry so much about reduced diversity. Once we're good enough at creating genetically manipulated babies we can recover any diversity we want in one generation.
It almost reminds me of ...me. "I don't need to study and get good at studying, I just need to get good at self-hypnosis and instruct my brain to absorb things faster and remember things better." Want to know how that turned out? It didn't.
Unfortunately, diseases typically run their course much faster than that meaning it would be possible for a disease to devastate a genetically homogenous population before they can raise the next generation and maintain population levels.