There wasn't migration to python because there's a total lack of anything to migrate to in python. It has only textbook examples as far as statistical models go. That's around 10% coverage and pretty much 0 coverage for all the abstractions and utility packages in R. Pretty much the same with machine learning (minus neural networks where it is strong). Python took some share from matlab and data processing tools but I don't see it retaining that. IMO it will lose it to Julia for the engineering & science people and to Scala for the data processing people.
The "second best language for everything" doesn't really work too well in the "data science" world.