> Python 2 features were reintroduced in basically every P3 version until at least 3.5

If they had just done this from the beginning there wouldn't even have been such upgrade drama in the first place... like, as an obvious example, removing u'' syntax for unicode strings immediately at 3.0 was just idiotic: if it weren't for some dumb decisions like that one there would have been almost no upgrade discontinuity at all (a la Ruby 2's Unicode reboot, which concerned a lot of people but was a nothing-burger next to the insanity of Python 3).