My takeaway from the article :
Debian has made the decision to ship Python 2 with Debian 10 (“buster”), which will be released in mid-2019; that means Python 2 will be supported in Debian well past its 2020 end of life. For Ubuntu, the plan for the 20.04 LTS release has Python 2 removed from the main repository. That means it will not be supported by Canonical, so the community will need to pick it up if it is to continue at that point.
Historically, KiCad devs would wait until KiCad no longer builds on several platforms before updating a dependency, I think we can see with gtk3 this causes a somewhat painful experience. The trade off is causing users a lot of upgrading work.
So there is a non-obvious “right time” to move to Py3, my gut tells me between now and v6 release is the time. It might be nice if KiCad was ahead of the curve on this one, even though I am loathe to rewrite a lot of scripts for Py3, it has to be done sometime.
A clear heads up for users is needed, KiCad should announce that v6 will have Py3 support and you have all got 2+ years to get ready. Nightlies post 5.1 should have Py3 enabled by default.