This is dangerous logic, because you're not comparing it to anything. For instance one argument I've seen for the existence of a god was the fact that many constants and other seemingly 'magic numbers' of our universe are set just as is required to maintain life as we know it. The problem there is that assuming this is true, it's still meaningless since the only way we could ever come to observe this fact was if it was true. This observation is known as the anthropic principle [1].
Basically considering the merit or probability of something happening when you would be unable to observe it not happening is impossible. You could say you're comparing it against the failure of other groups, but this is probably somewhat disingenuous as I think it's reasonable to hypothesize that the oldest persistent ethnic group is likely some group within Africa neither you or I have ever heard of, and you'd probably be unlikely to praise their longevity and persistence in and of itself, even if it too was likely ripe with strife throughout time.