My bet is that Alzheimer's disease is a viral defense run amok. Most cells simply destroy themselves to defend against viruses, but most neurons need to last a lifetime. Therefore they must have a way of choking off viral replication and signaling their neighbors to make their own preparations. This fits with the metabolic collapse and with the fact that amyloid beta spreads to nearby cells and propagates the collapse. The mystery is why it becomes self-reinforcing.