The reason for the but is that if you are trying to merely decompose into atoms, then you have no reason to aim to create atoms that are maximally reusable in creating other atoms. Indeed not creating them makes for more local simplicity.

When reusability conflicts with local simplicity (and they very often do), saying that reusability generally wins is based on some value system. Without a value system to reason from, your conclusion is not just logic. I think that the concept of intellectual compression captures the value system quite well. I think that the atomic hypothesis captures what you do, but misses why you do it, and cannot resolve that conflict.