Assuming that an older littley who is BF doesn't drink from a cup or satisfies his thirst with breastmilk is wrong. CHildren who bf until naturally weaned eat a full, balanced diet, drink from a cup and have a normal life like every other child. You can leave them with others, they can form relationships with others, go to nursery or even school, have friends round, visit granny, whatever. They just have another thing that they do in the course of their routine.
Often ebf is only in the morning and at night and doesn't "interfere" with a child's day to day living, only enhances it. Sometimes, when a child is sick or fretful it is the only way to help settle them/get some fluid into them.
It's very wrong to judge people because they do something differently to you. I have known children with bottles and dummies at 3 or 4, older children with comfort blankies which they suck, thumb suckers and more, and I do not judge either the child or the parent. There is nothing wrong with giving a child the comfort that he craves in the manner that he craves it. I sucked my thumb until adulthood so did my brother, we are both normal, well adjusted human beings who have suffered no trauma from our protracted thumb sucking, just as none of my children have any ill effects from prolonged breast feeding. In fact, bf helped my children to take many things in their stride which could have caused more upset, as it was a constant that they could rely on.
AS far as tooth decay goes, I know very little about dentistry but all my children have perfect teeth, even my 20 year old, and they all fed through the night on demand until they weaned. But perhaps that says more about the fact that they were never really allowed much to eat in the way of rubbish than anything else.