Forget Alzheimer's specifically for the moment, we don't know enough about it. In general, early detection means cheaper and more effective treatment, so the interests of the insurer and the patient are aligned, and the profit incentive helps both.
Back to Alzheimer's, it looks like there are things people can do to lower their risk, so knowing early on that you are at a higher risk may be beneficial. Perhaps postponing symptoms until after something else kills you, or perhaps avoiding it entirely. Maybe nothing will help, but at least you have the time to plan ahead for when you can't plan anymore.
As for what's next, dementia isn't the biggest killer, it's heart failure. Eliminate dementia, and people's hearts will still stop.
Don't forget that legally prescribed drugs kill a lot of people.
I've been conducting a bit of a study myself. Of course, it's not a big study. But in my peer group, of the men in their early 70's, most are on statins and other maintenance drugs like ACE inhibitors, ARBs, and beta-blockers All of them look. pretty old and wrinkled, all are impotent. Note, this has been learned in conversations where they express "those days are long gone" referring to romantic interludes.
Wife has been damaged by maintenance drugs. But she still takes them religiously.
On the other hand, I take no maintenance drugs at all. I only doctor for acute things, like when I managed to get Lyme a few years back.
I'm surely not perfect. I have a lot of old Ice Hockey injuries that are presenting with arthritis. Aconite patches knock that down. But I'm still quite capable, if you catch my drift. People are drugging the bejeezus out of themselves, and just creating new problems. And don't get me started on how actual antipsychotics are being mainstreamed for women. (things like Rexulti, Abilify, Vraylar, Latuda) These numb her mind no doubt, but they also present with side effects like Tardive diskenesia, which can be permanent and sometimes quite disturbing, weight gain, sexual dysfunction, "unusual urges" and remarkably - suicide.
Don't worry though, there are dugs for the jerky movements. Oddly enough, they can also cause tardive dyskinesia.
Am I a one-off? Could be. Could be my peer group somehow preselects for old unhealthy dudes. But I learned long ago to trust doctors and Pharma like I trust any other drug pushers.
And somewhere between nuts like Robert Kennedy, and the people on a gazillion maintenance drugs, there needs to be a middle ground.