I am sorry, there are many aspects that I have to disagree with you. These are historical perspectives as laid out by many historians, although one may find different historical hues from differing historians.
To say that Harappan people just vanished out of thin air and no trace of them has been left is factually wrong. Their legacy also remains in many forms even to this day.
After 1900 BC there were no longer references to Meluhha in Mesopotamian writings, and no Indus seals are found in Mesopotamia after that date. Whatever the reasons of their diminishing trade may it be changing course of river, drought, Aryan invasion, decease, earthquakes or whatever, the fact remains that their cities shrank in the second millennium BC, yes, but people still lived in places like Harappa long after that.
The continuing prosperity of the bigger cities, like Harappa and Mohenjo-Daro, may have made them grow too large and unwieldy to administer, and so groups may have split off into smaller settlements. But those settlements were held together by their common culture. Perhaps the dispersal was a way of providing flexibility to deal with the reasons I explained above.
Some of the aspects that we can still trace their legacy to are the commerce routes they developed. Traders from the highlands of Pakistans Baluchistan and northern Afghanistan brought in copper, tin and lapis lazuli. The Makran and southern coasts of Pakistan provided decorative shells. Timber was floated down the rivers from the Himalayas and gold from southern Central Asia. Skilled Harappan artisans and specialized craftsmen turned such raw materials into useful and beautiful products for regional distribution andas finds elsewhere have shownfor export by land and sea to Mesopotamia, Persia and Central Asia.
The southern states controlled the sea trade, just as Karachi does today. Ships from Meluhha [the Mesopotamian name for the Harappan nation] regularly sailed from Lothal near modern-day Karachi, Pakistan for the ports of Babylon. And they evidently made stops all along the way, as I mentioned earlier that Indus River seals have been found in Oman, Abu Dhabi and Bahrain as well.
The modern city of Peshawar lies on what is thought to have been one of the Harappans main overland trade routes. That route is now a major highway that constitutes the eastern approach to the Khyber Pass and links the northwestern Indus Plain to the highlands of Afghanistan and Central Asia. An old branch of the route runs from Peshawar south into rugged tribal territory, through the modern towns of Pakistans Kohat and Bannu and the foothills of the Pakistans Sulaiman Mountains, and on down across the Gomal Plain to the early Harappan site of Rehman Dheri, where an important excavation was conducted from 1976 to 1980.
Hindu Aryan culture was distinctly different than the culture of the Indus Valley Civilization, which was not Hindu and yes were not Muslim either.
The IVC was mostly restricted to the valley of the Indushowever at the tail end of the civilization some of it dispersed into areas beyond the IVC.
The 3 Major IVC cities are all located in Pakistan. One in Western Pakistan, one in South and the last one approx in central Pakistan.
Current day India didnt exist during IVC, and their religion had nothing to do with IVC, and major IVC settlements are not even located in India. However, Indians still refer to India as the Home of Indus Valley Civilization which is indeed surprising.
Indus Valley Civilizations legacy is linked to Pakistan and it cannot be denied, because various peoples after their decline ruled or invaded the area.
Therefore, we the people of Pakistan rightly claim ourselves to be the scions of and holders of Indus Valley Civilization.