If you knew for sure they had it, and took no medications for it ...... they will do exactly what they're doing now .. slowly (or rapidly, depending on the dementia) deteriorating.
You will find that most people are on SOME kind of medication for SOMETHING. Blood pressure pills, fluid pills, thyroid pills, cholesterol pills, you name it, there's a pill for it .. if there isnt' a pill, there's a potion, and if in doubt, an injection.
THESE pills & potions keep people alive, probably for longer than they should be.
Typically, Alzheimers or any kind of dementia is usually left untreated, except in the early/mid stages with the options of aricept, namenda etc, which only puts of the inevitable.
Bit sad eh?