They did help things. But it took a long time for people to convert to them. There is evidence in some places are loop stirrups in the first century CE, but really it only became used after the Avars introduced it, at least to the West. And many cultures didn't use it or only did so sparingly. This is probably due to military conservatism and pride, but even so it wasn't a battle winning device. For example, it is likely that the Sassanians, despite contact with the Avars and a strong tradition of horse archery (see Aymar's post above about how it helped archers), did not likely use stirrups, and if they did it was probably not common.