I'd add that for the bulk of people writing software to support science, programming practices are among the least of their concerns. A data analysis program supporting a particular research project is more akin to a mathematical formula than to a piece of software engineering. Upgrading it to Python 3 would be about as motivating as going back and updating your equations in a research paper because a new font became available.

Moving your current/new development to Python 3, to a scientist, sounds a lot like "So there's this new font. It sort of works like the old one. No it doesn't let you write any new equations, and it may or may not complain about being used near papers written in the older font. We know you don't particularly like fussing with typesetting, but how about you invest in this maybe-compatible thing instead of spending your time doing your science? We promise you, the professional typesetters have moved beyond that stodgy Yourfont2.7 you've been using to write your papers."