The brain is a dynamic system, and a poorly understood one at that. It relies on active processes in order to not degenerate (likely the main reason why sleep deprivation is eventually fatal). Science is still some way from cataloguing all the processes involved, never mind fully understanding their workings.

Alzheimer's isn't a single-point-of-failure disease, more like a dysregulation of maintenance. So there are lots of things that can tip the odds one way or another, and we may get a lot better at prevention - to the point of reducing incidence maybe five-fold, or delaying its symptomatic onset to beyond the relevant lifespan - but the idea that there might be a single "cure" seems like wishful thinking.

It may also prove to be that, like a spinning-top, once things start to go off-axis it may be very difficult to restore the original stable state, and at most we can just slow it down a bit.

Perhaps more uncomfortable, if most of the effective interventions, risk-reduction-wise, are social/lifestyle related, that has implications for whether people have the agency to access those things.

(Time/energy/facilities/resources for exercise/active lifestyle being an obvious case in point, which is already well known to have a moderate protective effect).