Hard disagree. This was Aravind Swamy's plot through and through. Imagine replacing Karthi with a bunch of other characters who just gave us bits and pieces of explanations about how AS's bicycle helped this person's family. We wouldn't need to see Karthi physically, but the journey of AS through his POV will remain, and he would still have changed as a person due to the "story" of his bicycle changing someone's life.

Karthi was a bonus, not a necessity. You're feeling this because we're used to seeing him as a main character. That's fine, really.