This is a very poor take. My family has been affected by the disease. If I get it I would join research as it could potentially help my kid and other family members.

This is how science works. There are studies all over the US which involve existing patient and many join to help themselves and those who will come after.

Taking an optional test is voluntary.

Entering a study for people who tested positively would also be voluntary.

Anecdotal responses in this topic indicate there may be some mitigatimg medicines.

Demanding a root cause before doing anything is silly. Finding root cause in biological systems is incredibly difficult; there's value in finding symptoms and managing symptoms without finding a root cause. And dangers of focusing too much on any one indicator.

That's not to say I suggest everyone be screened for everything. If there are limited mitigations, it's not appropriate for most people to be screened. But even if there are no medical mitigations, you might put your affairs in order with appropriate urgency if you knew your cognition would be much reduced in the next 5-10 years.