It works only if you've also managed to completely erase the victim's identity and also the knowledge of said genocide. It's pretty much an all or nothing action, if you fail then you're pretty much condemned as a genocidal warlord unless you manage to be the victor or let enough time pass to make people forget about the gravitas of the actions that you've made
and even if you do manage to succeed, then you've only killed an ethnic group out of a hundred. I'll give you an example of a successful genocide that also made people forgot about it . An empire that managed to be (almost) completely erased from history, like the Olmecs did, was the Western Xia Kingdom. It was razed by the
Mongols, leaving Uighurs squatting on its land. Even China today had barely any lick of idea on these guys because the Mongols had thoroughly killed off their identity and their population. And you know what? Nobody, and i mean nobody today is shedding any tear on them. They're now just a footnote in some specific history books on Chinese history.
So, it's true that Genocide works, but a successful Genocide is hard. And the rewards for doing so is just a reduction in the overall human genome, alongside the death of its culture.
Killing an identity isn't as easy as putting 6 gorrillion jews in a bakery oven, especially when we have more writing mediums than we've had in the last century.