Neither Medea (if Assyria fell) nor Assyria (if Assyria didn't fall were any slouches, as far as power went. Both were capable, for a few centuries at least, at holding the line against most incursions from Central Asia. And we're not in the midst of an episode of global cooling that will FORCE mass migration of nomads against empires at this point in time, I don't think. If the Massagatae DID conquer Medea or Assyria, like the Dahae conquering Selucid Persia, they would adopt most of the conquered's customs in order to rule them. Because a more civilized neighbor has a better chance at conquest, I suspect Bactria would finally take over. Or if Medea or Assyria really lasted long enough to become decadent, Mauryan India might finally fill the power vacuum all the way to the Mediteranean Sea.