The Holocaust failed because Hitler chose genocide instead of forced assimilation. Genocide almost always fails at wiping people groups out, because some will always survive and carry on their people and culture. Assimilation is a cultural genocide; strip them of their ethnic and cultural identity, and their group will cease to exist. All the ethnic groups that got wiped out or are on the verge of extinction are so because of Assimilation, from the indigenous Central and South Americans getting absorbed into Latin American Mestizos, or the Chams of Chiêm Thành and Khmer Krom being forcefully assimilated into Vietnamese culture, to the northern Bách Việt having been fully sinicized and becoming the modern-day Cantonese, or the Jomon and Ainu being absorbed into the Japanese; I could go on and on. Plus, the Jewish population in Germany was small enough that if forceful ethnic and cultural assimilation were imposed, the Jews wouldn't make much of a genetic impact on the ethnic Germans.
TLDR: Assimilation is more effective at wiping out a race than genocide
