libc isn't going to break, full stop.
However, what *is* a concern is a more general case of something which we
did see in the last major upgrade. Remember the wtmp format change?
Remember the GCC exception-handler errors, back when libgcc was always
statically linked? This is a special case of a more general problem, which
is that if some library code manages some shared resource, and more than
one variety of it gets to run in the same address space (as happens with
static linking, and with differing-soname shared linking), and those
varieties have different expectations of some shared resource, things will
Unix doesn't really have a good answer to this other than `soname breaks
that affect shared libraries that are linked both into libraries and into
the users of those libraries should force relinking of all those libraries
at once, or changes of the intermediate libraries' sonames', both of which
avoid having multiple copies of different versions of the same shared
libraries in the same address space at once.
Neither of these situations are exactly ideal, but as far as I know
nobody's thought of a better way. (Has anyone?)