In the 1930's, an astronomer predicted that cyclic changes in the Earth's orbit could have caused the Ice Ages. Notice what this was: it was a prediction about the past. It was legitimately a prediction, because it was about evidence that no one possessed. So, the theory was in principle falsifiable, even though no one was sure that we would ever find the necessary evidence.
In the 1960's, a geological method was found that measured ancient climates. The predictions about the past nicely matched the new data. The Milankovitch theory is now good science - and it always was.