I think it's recommended for distros to modify the shebang to /usr/bin/env python3 for script that are supposed to run under python 3, although this does pose a problem for scripts that want to run under both without modification. If the distro ships with python3 and most of the system scripts run under python3 I'd say its the default.Ubuntu for one is planning on not shipping a python2 or /usr/bin/python binary by default(although it will still be in repos) eventually.