The numbers are pretty bad around the whole thing. Even if you are generous and allow for for extra survivors of the Decimation, the number of mutants that retained their powers is no more than 0.01-0.02% (based on an overestimation of mutants keeping their powers from published numbers (400), the stated remaining mutant population before Decimation as "millions" (2 million), and the percentage that lost their powers in Decimation (originally published as "90%", later amended to "over 99%"). So given the pre-Genosha genocide, worldwide populations of mutants was at least 18,000,000 (there were probably nonmutants in Genosha, but we don't have any numbers for them), but probably not much more than that. I suspect the New X-Men writers pulled the 16 million number out of their desire to have it parallel the Holocaust in magnitude, but it'd be nice if that was actually dealt with somewhere. If 90% of a population with worldwide impact, that populated an actual nation just disappeared with physical evidence of their remains that still needed to be buried (remember Necrosha?), it would have an impact beyond being the hook for a book, and it just seems sloppy and lazy to handwave the repercussions of that away. Like, the Holocaust has social, political, and personal ramifications today. How fucked up is the Marvel Universe that this is a metaplot point that impacted lots of future stories and no one cares?