My sense is that smaller libraries are more able to evolve at their own place - e.g. the separation of django-rest from django core, or languages moving more things out of their standard library. Pre-1.0 is the easy part, we'll see how Go does is 5 or 10 years once it has a base of programs to maintain compatibility with - I predict it will find it harder to evolve and its library will fall behind.
At scales above the absolutely trivial I think libraries evolve more flexibly based on social rather than technical considerations. It's about not pissing people off when you change the interface. Even something as complex as Go was able to make incompatible changes pre-1.0.