The main criticism against the possibility of a huge increase in life expectancy in the next few decades is that the human body is so complex, and there are so many ways in which it can break down, that there's no way we'll have figured out a way to repair all those possible forms of damage in just a few decades.
IIRC, Aubrey de Grey's response to that criticism is that repairing damage is only one way to increase life expectancy; it's also possible to keep the body young enough so that it will never break down and thus won't need to be repaired (for most people, anyway; a small fraction will simply have bad luck). Stopping aging, if it's a relatively simple thing to do, would therefore allow life prolongation without having a full understanding of human biology.