I don't think your theory is right (I simply don't think it has been given enough polish), but even so, the other side of the coin is that you can relatively easily hack in temporary migration paths. For instance, a function can examine the parameters it is given and convert them to the latest API, spewing out a warning.
Backwards compatibility is mostly an attitude problem.
Yeah if all you have is straight calls to functions, as soon as you do stuff like assign function to a variable, method calls you need to do sophisticated code analysis and that's not even touching the untraceable stuff like string/dynamic access, monkeypatching, etc. etc.
Refactoring in python is bad even if you restrict yourself to "sane" code (no metaprogramming and abusing dynamic stuff so the tools can follow what you're doing) if you need something that will work for everything out there it's just impossible to do.
Backwards compatibility is mostly an attitude problem.