Self interest is the driving force of individual action. The sum of individual actions makes up a society. Therefore, self interest is the driving force of society. Will it lead to downfall? It depends on your definition of downfall. If you view a society where individuals are free to act in their self interests as a good thing, you'd say no. If you view a society where power and coercion are used to force people to act against their self interest as a good thing, then you'd say yes.

You should check out my series on Freedom. One of the topics I plan on getting into revolves around the concerns you expressed in your post.