You actually answered the OP's question. Puranic Hinduism pretty much castrated this country. Everything was either maya or Karma, what incentive was there for people to strive? The golden age of Hinduism that is often mentioned is more accurately the golden age of Buddhism though the rulers, the Guptas happened to be Hindu. As Buddhism declined & puranic Hinduism took hold, India slipped into a morass.By the end of the Gupta period, caste was firmly entrenched & that pretty much doomed "India". The Kshatriyas were pretty useless in fighting & essentially needed to count on the Shudras to provide the numbers. What incentive was there for the Shudras to fight against one bunch of oppressors when the other bunch, already existing, were doing the same oppression for years? Stupid clan rules, rigidly followed, were taken to such extremes that artisans were not even allowed to change their tools across generations disincentivising any innovation. When "thinking" was made the monopoly of the few, it was the death blow whereby the intelligence of the large majority of people was rendered unusable guaranteeing a rapid civilisational decline. Regardless of individual brilliance or philosophical genius of a few, Hindu India was pretty much doomed by puranic Hinduism. The Bhakthi movement which revived Hinduism for some time was a far cry from the extraordinary metaphysical postulations of the Upanishads, a dumbing down if you will. Islamic invasions only hastened that decline by wiping out the "thinkers" in the cities. Hinduism remained in a slump, dying & decaying slowly till ironically the advent of the British helped breathe new energy into it..