This question is hypothetical because it's never going to happen, so there is no question of answering such questions. Please take a good read at Sachhar Committee report of UPA I , they have answered this question empirically. Right now Hindu population is approximately 80% and Muslims are 14%. read more
Yes for sure India will end up having a Muslim majority even though having Hindu majority in current but in future it will be a muslim majority like Afghanistan and Cambodia. The effects on the Hindu will be more and India will become a war-torn country just like Afghanistan or Syria. read more