My own observations:
People developing dementia can hide the symptoms for years. Everyone forgets things. Everyone does something weird once in a while. There's something called Paramnesia where you 'remember' things that haven't actually happened (there's an example of an American President who recounted the plot of a movie he was in as something he had done in real life to a group of veterans.)
It's when things collapse that people get diagnosed. I work with people who on most occassions can pass; unless you know that the little boy they are looking for is in his 50s, that they aren't allowed to keep money in their room as they look for their purse, that this is the third time in an hour that you've had the exact same conversation with them.
People can die in very short times after dementia affects their mind. What ever is causing the damage to the parts of the brain involved in memory, emotion, and thought can also damage the parts of the brain involved in breathing, swallowing, and body temperature regulation.
There is no good death from the point of view of the loved ones.