Combining X+Y in the way that you have described is anything but
mechanical: it is a task which typically takes a skilled programmer a
great amount of time and thought. Different programmers might do it
in different ways. I'm not referring here to the work done by ld, but
to the process of building a new program which has libfoo as a
component.
Additionally, the program ultimately delivered to the user isn't X
with some minor bits of Y. It contains big chunks of Y -- one per
function used, at least -- directly copied. Just being in a different
memory space isn't enough to change the relationship between the
creative parts of the works. The program vim encompasses a copy of
libc.
-Brian