Sadly that is something many people can't rely upon. I am not that old, and have a tendency to get pretty dehydrated before i get thirsty at all. Old people are famous for getting so dehydrated that they get delirious - ask any nurse working in a hospital, they will agree that this is the case. With summer temperatures rising this will hit a lot more people of the already affected groups, and additional groups which are not sensitized yet towards that issue.
I agree that most people do not need to drink 3 liters of water per day. But what happens is that people aim for that amount and then land somewhere around 1.5-2 liters / day, which is a pretty good amount for normal days, and they will be more alert towards their water intake on hot days, which prevents deaths.
