The guy saying "their kind will go extinct" is referring to the common but sadly not as true as we'd like concept that bigots are disproportionately old and that over time their ideas will die out as they do. We need look no further than the groups Hillary discusses in her speech here to see that this is by no means a universal law.
But isn't the alt-right truly a "fringe" group? Isn't the tide of generational Western culture as a whole leaning more and more progressive?