There's also many old people that are so demented they're just sitting in a chair drooling, totally unaware of their surroundings. They receive as much care as they can get but it's not really improving their quality of life. They basically exist only as a reminder for the people that knew them.

I do think that euthanasia would make sense in these cases. The real person, their mind, has died a long time ago, it's just their body that lives on.

Of course the decision should be up to these people (while they're still capacitated) and their family. It shouldn't be imposed on them. But personally I wouldn't want to "live" like that.