I think it depends on the mother. Some moms respond really well to the pump and others don't. Even the moms who pump easily see a decrease in supply as time goes on though. The pump just isn't as good as getting milk out as the baby!
For me, if I could nurse the baby then I would not bother. What an awful lot of work when I could cut out the middle step and just nurse my child! I feel that if the child still needs to suck, and still needs breastmilk, then they still need to nurse. We're mammals, and that's what mammals do.
Nursing isn't just about the milk - there are other reasons to nurse a child as they get older. It facilitates normal jaw and face development (potentially saving trips to the orthodontist!), it helps with emotional developments, it helps with brain development, and it is a fantastic way to soothe and connect with a child.
At 4 months I could not imagine nursing a 9 month old even (my DS is now 9 months old). At 4 months it was still about nursing every 2 hours and I had no idea how my nursing relationship with my baby would change as he aged. Now I look forward to nursing a toddler. I can't wait until he has a word for nursing (right now he signs 'milk'). Even at 9 months I love how nursing can help calm and soothe him after a tumble.
There is absolutely no reason to think that nursing an older child will be harmful to them in any way. I hear that occasionally - that nursing will 'damage' a child. Nursing is normal! If the child wants to keep nursing, and the mother is agreeable, then there is no harm, only good.
What has got you thinking about the future, PollyC? :)