2018 and 2019 is when you'll start to see all the distros with an enterprise support lifecycle (RHEL, Debian, Ubuntu LTS, SLES, probably others I'm forgetting) start to move more heavily to Python 3, since Python 2's EOL is in 2020. As your link notes, Ubuntu and Debian are trying to make their next long-term supported stable releases use 3.x as the default Python.
Looking to the following iteration, any distro releasing in 2020 with Python 2.7 as default and a support lifecycle greater than 6-9 months, doing so after 2020 regardless of lifecycle, will be irresponsible. I doubt any of the major ones will overlook this, not even those which target hobbyists instead of enterprises.
(Disclaimer: While I am a Debian developer, I have no personal involvement in this transition for Debian or any other distro.)