Mono or Polytheism?

We have been accustomed to thinking that monotheistic religions are more advanced forms of religion. But who ever said so? Polytheistic religions certainly have fewer difficulties in explaining the origin of evil. It seems to me that the tendency to reduce the number of deities to a single, irreducible one—standing outside the system and taking responsibility for everything—risks impoverishing our perception of the universe and leads us away from, rather than toward, a proper understanding of its complexities.