The Muslim percentage in India has risen significantly since independence, as well as since the British conquest. In 1951, India was 84.1% Hindu, a figure which has only decreased. The farther back you go, the more Hindu.
OP asks about South Asia, not India in its post-Partition boundaries.
If an Indian empire (surviving restructured Marathas, maybe) modernized in the early 19th century, and followed the demographic transition at the same time that Europe did, it would be a super-rich country with only 300-500 million people, meaning that <15% of the country, <45 million could immigrate to countries across the world easily.
Why would it only have 300-500 million people? South Asia had 200 million in 1800, and has had a factor-of-8 population growth since, which isn't that much higher than England's growth since 1800 (8.3 million -> 53 million). And Northern Europe had had significant population growth in the Early Modern era, whereas the Mughal Empire already had 150 million people in 1600.