>Like with any other system where some old version just won't go away, the ones who absolutely will not switch to Python 3 will eventually (literally) go extinct.
And yet there are tons of lines on COBOL in active use today. What makes you think 2.7 code, which powers tons of businesses, is going anywhere?
At some point in the future the cost of maintenance vs. cost of rewriting/upgrading will be so much in favor of the latter that businesses will make the (correct) choice to "go with the times" instead of spending an ever increasing amount of money and time on maintaining a code base `x` years old written in a language not officially supported/extended for `y` years, experts for which are increasingly rare (and thus expensive).
What we can do, as the people who live in the present, is educate programming/python newbies to use the newer/better Python 3, instead of clinging to something that will soon lose official support (for AFAIK good reasons).