but when they are that bad that they dont know you, generally you are not the one doing that kind of care anyway - the washing, toilet, etc etc.
when they are ok, just old, you visit the home in order to spend some time with them, to talk, have a laugh, play a game, just be there for company - you dont start bathing them, feeding them etc
so when they no longer know you, the purpose and focus of your visits - quality time - no longer exists, as they have no idea who you are and whether you are there or not - they still get all their needs taken care of - food, bath, etc etc, by the staff
all that happens is the family feel devastated, hurt and distraught, and the person in question feels nothing, no hurt, no loss, no loneliness etc, the family are forgotten almost as soon as they leave.
family still pay for care, ensure they have all they need, and presumably check on them physically every so often, but weekly 2-4hr visits etc etc would just be painful - especially if it was over a period of many years - a few months then i would think most people would be there.
i would go for as long they knew me, and knew they had company - because i would want to see them, but also i know they would be upset if i didn't, they would want to see me too, and i'd obviously provide whatever else they needed, but i could not bear to watch my parents in such as severe state, it would destroy me ... in fact its making me feel a bit tearful right now just thinking about it ...
fortunately unless they both live to about 90-95 i can't see that happening really (hopefully), as they are both still active etc at 74 and 85 - a wee bit forgetful at times, but nothing unusual