> For a disease which (to my knowledge) can’t be slowed down or reversed, I think it’s a fair question why we would want to detect Alzheimer’s. Maybe there are other reasons, but my suspicion is that we will be able to, and an easy detection method significantly widens the pool of subjects to study later on.

Your point at the end is essentially correct. There's a couple of reasons that come to my mind:

Early detection lets us test cures more quickly. You can see if the treatment is working without waiting 30 years for symptoms to develop or not. If prevention is all that works, we can verify lifestyle changes, again without having to wait 30 years for symptoms to develop.

Early detection means there's more of a chance of any future treatment succeeding and the patient returning to a normal life. Think of early detection of cancer or heart disease meaning you can be treated with less risky medication and procedures and minimise the damage being done.