The Holocaust - or a genocide very like it - can and will happen again. The best that any of us can say is 'not on my watch'.
Perhaps the only silver lining of the Holocaust is that it now stands as a recent, vivid and perfect example of how racist policies can go too far. It's a very inconvenient entry in the historical record, for those who would do it all again.
Much better to make it all go away.
Post-Holocaust antisemitism has had to wrap itself in knots, in order to be taken seriously by anybody. "You want to include Jews in our society? Then YOU are antisemitic, not me!" They cry, continuing in the face of incredulous frowns, "Because if you don't separate them, then societal diversity will make them lose their culture! Dilute their bloodline! YOU WILL HAVE WIPED THE JEWS FROM THE EARTH, JUST BY INCLUDING THEM!!"
And for 'Jews', please feel free to insert any other race different to the speaker. Or indeed, those of a different creed, political leaning or whatever else comes to mind.
For a moment there, such twisted logic sounds superficially reasonable, until you realize that the natural outcome would be ghettos, camps or sealed off states. Anything which a) emphasizes race like it's not a meaningless distinction and b) promotes segregation. It makes it sound like concentration camps are a good idea.
Many Holocaust Deniers hail from neo-Nazi, far-right or KKK-type backgrounds. For those members with keen intelligence, then the Holocaust can be used as a means to ensure that it happens again. All they have to do is persuade enough people that it didn't happen in the first place, or it was exaggerated, in order to allow its perceived victims to capitalize upon survivor pity.
If a consensus could be reached to agree upon this, then mainstream society would naturally feel angry about the inherent duplicity. We would all turn against the Jews (and other 'undesirable' groups) and turn the other cheek, as the machinery of exclusion through to extermination kicked in again.
It's a sad indictment of human nature that such a reading is probably correct. Society can all too easily turn against a sector of itself. History has provided countless examples of that over and over again. Currently the Holocaust - its place in living memory, its wealth of evidence, and majority acceptance that it did indeed happen - stands between us and the next time.
No wonder the more insidious of antisemitic, and other racist, movements would have us deny it. Carefully inserted doubt shoots holes in the 'never again', but only if we let it.