Could a human survive solely on human milk into adulthood?
This is an interesting thought experiment. The short answer is no, not really. Here's why:
I looked at the constituents of human breast milk, which are found here: Food and nutrition bulletin
In order for an adult to get 2000 calories per day, they would have to ingest about 3 liters of breast milk in a 24 hour period. This is certainly within the average person's capability. The calcium, carbohydrate, and vitamin C levels in breast milk are all close enough to the recommended daily allowance (RDA) set by the FDA to sustain a human. The problem comes in when you look at zinc,potassium, iron, B vitamin, sodium, and to a lesser extent protein content. All of these nutrients are too low in breast milk to sustain an adult (there's only 20-50% of the RDA for these nutrients present in breast milk). Another problem is that the fat content is nearly double what an adult needs, and there's 2.5 times more vitamin A than the RDA. High vitamin A intake for long periods can actually lead to toxicity, which can cause a host of problems. We also know there isn't enough iron in breast milk for an adult, since we see older babies who are doing extended breast feeding turn up with iron deficiencies. If there's not enough iron for them, then we know it won't be enough for adults.
If an person attempted to live on breast milk alone into adulthood, they'd be obese, anemic, and probably have bone, muscle, and nerve problems due to vitamin and mineral deficiencies. They might also develop hypervitaminosis A. Overall, breast milk would be great for a baby but not so much for an adult to live on.
Now, another interesting thing to consider is that there is enough nutrition in breast milk to sustain an adult for a shorter period of time - say a few months. However, since the average woman produces between 700 and 800 mL of breast milk per day, it would take 4 women to make enough milk.