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.