It’s pretty straight forward to select away almost all cases of Alzheimer’s by simply not procreating if you are positive for APOEε4, for example

No suffering required

I disagree with your assertion that you can deny procreation to swathes of the population without causing suffering.

Sure. There’s various ways to influence the gene pool and most are not great ideas on a large scale. Enforced non-procreation for people with “undesirable” genes is a dark road to go down. Especially if we have other ways to treat these problems.

In the very long term the lack of natural selection could have a noticeable degrading effect on the human genome, but it’s a slow process of drift. Since gene editing will probably be more common eventually I don’t see it as a major concern. We left the brutal mechanisms of nature behind, and we already have the means to replace them. It’s right that we approach the topic of genetic manipulation with caution, focusing on clear cases of extreme need. We’re fine for the foreseeable future.

Bridging the gap with eugenics or the like just isn’t necessary, and is likely to cause societal problems far outweighing any advantage.

That’s not natural selection.

Natural selection was always the cruelest way to manage disease. It requires us to be struggling to survive. When a significant proportion of humankind have died from a disease, we may evolve a resistence to it, but that’s a brutal price to pay. We’re past that, and good riddance to it.

But you chose a strange place to bring that up. A drug to prevent Alzheimers won’t have much effect on natural selection, since a susceptibility to Alzheimers would very rarely prevent successful reproduction. Alzheimers is strongly tied to the ageing process, which is probably an evolved characteristic in itself. Natural selection was never a friend to the elderly.