This is the "problem" with not having things controlled by some central figure. Following the road example, the Dept of Transportation would simply open the new road, and close the old. You wouldn't have a chance to continue traveling down the old. This happens (to a large extent) with Java. Oracle opens the new road, then stops fixing potholes on the old one. You're not "forced" to move over, but eventually you stop wanting to fix your suspension.

That said, who is central enough to put an "end" date on Python and "force" people to move over?