What are the differences between the following 2 sentences?
A: 僕は小さい字がだんだん読めないようになった。(Lit: I gradually became unable to read small characters.)
B: 僕は小さい字がだんだん読めなくなってきた。(Lit: I gradually became unable to read small characters.)
A is less natural than B, because だんだん, which stands for a gradual change, doesn't really get along with なった, which is an instantaneous change.
But it sounds like approximation of だんだん読めないようになっていった. In this case, the difference is, なってきた means that the change continues from a certain point in the past to the moment of speech, while なっていった means that the change started but it doesn't refer to whether it has reached the moment of speech.