There are two things you could mean when you say 'reductionism is right'. That reality is reductionist in the "big thing = small thing + small thing" sense, or that reductionist explanations are better by fiat.

Reality is probably reductionist. I won't assign perfect certainty, but reductionist reality is simpler than magical reality.

As it currently stands, we don't have a complete theory of reality, so the only criteria we can judge theories is that they 1) are accurate, 2) are simple.

I am not arguing about the rightness or wrongness of reductionism. Reductionism and contra-reductionism are containers, and they contain certain classes of explanations. Contra-reductionism conatins historical explanations, explaining the state of things by the interactions with outside forces, and reductionism contains predictive explanations, explaining the future behavior in terms of internal forces.