It's not only only about big libraries. Unless you do web-dev and can rally only on the currently developed libraries, from time to time you will get a specific, abandoned library. Even if there is just a single such library, Python 2.7 may be preferable.

Moreover, in machine learning Python 2.7 is still considered the default version of Python. (E.g.: some part of OpenCV need Python 2.7; until recently Spark supported only Python 2.7.)