The reason there's no cure for Alzheimer's is that nobody really knows what causes it.
The FDA approved the drug, but required Biogen to carry out a Phase 4 study to see if it actually works (they gave Biogen far far too long to carry out that study, but that's a different story).
So, there's a simple solution grounded in consumer law: if you buy something, and it doesn't do what you promised it would, you can get a refund.
Approve the drug, require a comprehensive Phase 4 trial, and require Biogen to provide a full refund unless the Phase 4 trial conclusively proves that it works.
Remember -- only 10% of anything can be in the top 10%.