I saw an enjoyable but historically rather questionable Bollywood movie complete with mega song and dance numbers called Mohenjo Daro. Rather implausibply, it showed the city being wiped out by a flood and the survivors moving from the Indus to the Ganges and becoming modern India’s progenitors.
While the flood bit may be overblown, it can’t be discounted. There was climate change and some of the folks did move east. But it was definitely not an overnight event, more like a long-drawn out event over decades or even centuries