To my mind this is a brilliant if unpopular analysis. I would really like to see the ability to import modules "from the past" but it seems extraordinarily difficult to implement that now that 2 and 3 have diverged so much. And one of our problems is exactly what you describe: a large, very-well-tested, 2.X library that we rely on is no longer maintained by anyone. So we'll have to port it ourselves, and surely introduce a bunch of stupid bugs. Grumble.

A saving grace is Python's historical emphasis on unit testing. That makes fore-porting things without horribly breaking them at least plausible. But still painful.