Cold water fish tend to be able to handle warm temperatures because even in cold water climates, the waters reach high seventies... If there's prolonged exposure to these high temperatures, their lifespan will be shortened, however, putting a tropical fish in cold waters would kill it much quicker... your argument makes no sense...
At first it may look like the snakehead will wipe everything out, but the ecosystem has a way of evening it out, yes there'll be less of the natives, but they won't be extinguished. And MN Rebel's point was that only a select few species could actually have a negative effect, so up north the more tropical species should be allowed to be kept...