"Never again" has been said in other contexts as well. E.g. slavery in the US. US slavery didn't exactly meet the definition of ethnic cleansing or genocide. The primary goal was economic.

Just because something isn't ethnic cleansing or genocide doesn't mean "never again" doesn't apply.

>fully developed humans

What's the definition of fully developed? Are physically and mentally disabled people fully developed? There have been large scale killings targeting them in particular. Are babies fully developed? There have been large scale killings that kill them.