People only have a problem with breastfeeding beyond a certain age because of the sexualisation of breasts. Otherwise what is the problem?
Well exactly. If you remove this from the equation there really is nothing to get upset about.
Milk (cow's) is a huge part of many small children's diets. Surely it's obvious that human milk, made for human children is a better fit for their nutritional needs?
A child, even a small one, and most definitely a 2+ child needs to learn to adapt to the world it lives in.
A world where we've lost sight of the original purpose of women's breasts and sexualise the innocent act of feeding? No actually, I'm going to challenge that, not make my child 'adapt'. 
So, fine if it is the parents choice, but not if the child is dictating to the parent.
Anyone who has breastfed their child will know the relationship is a 2 way street. A mother who does not want to feed cannot be forced too. Neither can a child.
As mentioned upthread, people who have an issue with extended feeding need to challenge themselves as to why. I'd hazard that most of it is down to societal views of who breasts are 'for' (the menz). There may also be some negativity from those who bottle fed and were made to feel bad for doing so.