Evolution isn't likely to happen in self-replicating robots. Evolution needs a continuous fitness landscape like DNA. A random bitflip is more likely to produce a fatal error than do something beneficial. Even worse if the code is encrypted before copying. There also aren't that many generations. At exponential growth, it only takes a few dozen generations to reach the maximum population limit and stop replicating. Not enough generations for evolution to happen.

Obviously the technical issues with self-replication are difficult, but nature was able to do it with relatively crude methods. No doubt we could eventually design something significantly better. One major advantage would be mass cooperation. Evolution has no incentive for individuals to cooperate with each other, but designed nanobots can specialize and benefit from economies of scale.