As someone w/ a condition that leads to chronic low supply who is still breastfeeding a 16 month old, it can be done. The issue is education. There is not enough education done for pediatricians, midwives, OBs, doctors, etc. Many LCs are great, but many still don't know about real low supply. Since man of the experts have the knowledge needed, it is very difficult for a struggling, but committed, breastfeeding mother to figure things out.
Fortunately, I had a great LC who recommended to a Pedi/LC combo who was a godsend and diagnosed me w/ Insufficient Glandular Tissue. Then I was able to come up with a plan for success, for my babies and me. I did have to use some formula to supplement and it was necessary for my kids to gain.
I think the second part of the issue is that people are so worried about offending each other no one gives the encouragement or information struggling BFing moms need. We, as a society, are too quick to say, "It's okay to quit. Happy mom means a happy baby." Often, that is crap. Often, women don't really have a low supply due to a medical reason, they aren't nursing enough, drinking/eating enough, getting enough rest and help, bad support and education, trying to pump instead of nursing, pediatricians jump to formula too quickly.