What cause the Decline of the Aryan's?

Answer 1

The development of Hinduism, (and then Christianity and eventually Islam) altered the original Vedic Aryan religion which spread from Persia and India to parts of Europe, the Middle East and Sri Lanka, As Hinduism developed, elements of the original Vedic belief system were incorporated into the modern faith and modified by contact with other groups.

In areas outside of India, conversions to Christianity and Islam were often forced on the people by the rulers of areas were the Aryan faith was practiced.