The question would be how do you justify that "objects with parts" actually exist,

A leg carries a body. Without a body the leg can't function. Without the leg the body can't walk. Obviously objects with parts exist as I just described an object with parts. Obviously the objects are individually different as they have different functions such that two bodies without legs can't walk, even when connected. Obviously the composite is different than a sum if the parts as it can perform a function as a whole that none of the parts can perform separately.

It is absurd to claim that there is no such thing as "objects with parts".

Who is to say that a soul does not have parts?