The Reich's, ah, demographic adjustment efforts most certainly contributed substantially to Germany's defeat. The machinery of extermination ultimately employed something on the order of a million men of fighting age, men that otherwise might have been employed at the front, in addition to all the materiel resources that were poured into the pursuit of genocide. Then there were the ancillary effects, the engendering of support for partisan resistance in the East (which ultimately tied down hundreds of thousands of troops that, again, might have proved real handylike at the front), and the hardening of the negotiating position on the part of the Western Allies.
And all this to target a handful of small, geographically isolated and concentrated, easily identified (and generally unpopular) minority groups. Now, imagine the kind of resources you'd have to devote to selectively exterminating 80-90% of the total population. You might as well hope to flap your arms and fly as rid society of the its <120 IQ segment.