They didn't have "all but" 2 clinical trials. The 2 clinical trials everyone is talking about are the pivotal trials which are the studies submitted to the FDA for approval. There were phase 1 trial(s) on healthy people before that for dose floors and ceilings and safety, phase 2 trial(s) for efficacy, and the two largest, most robust phase 3 trials for safety and efficacy which are used as the pivotal trials. This is normal drug development.
Yes, it boils down to risk vs benefit. Which is what healthcare in general boils down to. But again, you have to start somewhere with an orphan drug/disease. This to me, in my expertish (that is, non-neuro-specialist) opinion, is a fine place to start.
The nature of Alzheimers is prospective anyway. These patients will need to be followed, and I hope to see this drug given to early stage patients in a phase 4 trial post-approval, which some medications also are subject to