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This scenario discusses present-day Indonesia and Malaysia still being majority Hindu-Buddhist or a syncretic belief system in the present-day. Think Hindu Balinese culture across most of the Indonesian archipelago and Malay peninsula. I would argue that if the Buddhist Sri Vijaya Empire on Sumatra withstood or at least recovered quickly from the Indian Chola invasion of 1025, a power vacuum would not have allowed the first Muslim sultanates such as the Samudera Pasai Sultanate to take establish a strong foothold by the 13th century. Muslim traders from India or the Middle East might have settled in the region, but they wouldn't make up the majority of the population. Let me know if this scenario sounds plausible and what other factors would have caused this outcome.