Just because someone else stepped in at the end that does in no way mean that the deaths of millions did not change the way that people thought. While the holocaust did not succeed in completely destroying Judaism and the Jews who's to say that the trauma did not change Judaism into something unrecognizable? To put this into context Jews has been living in Poland, Germany Russia and the rest of eastern Europe for centuries. They had their own language, Yiddish, own style of music, Klezmer, and a very particular way of life. All of that was destroyed, and for the most part vanished without trace after the holocaust, hundreds of years of a culture gone. Further, the response of Judaism to this catastrophe caused it to change form in many ways. The Jews gathered together into a state in 1948, something that had not happened since the destruction of the Temple by Titus in 70. Hebrew was revived, which had not been a spoken language for thousands of years and so on. Now i know that this is something of a devils advocate response, but consider the impact that a genocide has on the mindset of a people, it might do a very good job of destroying their way of life and what made them distinctive.

It is arguable, but yes, some genocides have met their goals. According to Bauer, the Armenian genocide for instance, achieved its pragmatic goal of eliminating of removing the non-turks in the heart of the pan-Turkish empire they wanted to create. The genocide of Natives in South America by the Iberians is another example of an arguably successful genocide, because they were able to enslave and murder, while at the same time plundering anything of value. However, both of these examples are full scale total genocide, which is more of a modern phenomenon. Also, state sponsored genocide, the type that you're thinking of is a modern phenomenon as well. Not to mention things such as partial genocide and pogroms to add confusion to an answer.