Really? Why don't you explain to us, why Neanderthal died out? Oh, and since you stated this as a matter-of-fact, I am expecting you to provide unquestionable proof, not evidence which might suggest something. Go ahead, knock yourself out, Einstein!
So...it's your contention that the only reason a group of people can die out is that they lack spirituality? Interesting.
Let's start w/ your contention that they were unable to assimilate into other civilizations. Neanderthals died out about 25,000 years ago. What other civilizations at that time did they have a hard time assimilating into?