False dichotomy. There are unknown mutants who could be killed by the cloud without realizing it, and the Inhumans don't need to sacrifice the cloud, just contain it. But aww, that violates their ancient culture which is purely scientific.
Also, to fight rhetoric with rhetoric: there are those with unknown deadly peanut allergies that could trigger within them. The Inhumans want to let a fucking cloud of peanut gas run free into civilizations (the denser the better) to convert a fraction of a percent to their side.
You wanna talk about "the slow, painful death of an established population"? Mutants can't be born anymore. All that's left is all there will be. The Inhumans can still make more, selectively. Their culture is not suddenly destroyed if they change some of their practices (see: fucking every religion on Earth). We're talking about a repopulation attempt that has a side effect of genocide on a population that literally cannot repopulate.
Gee, I wonder who's the bad guy.