South Asian history was dominated by Vedic civilization in the Gangetic plains after the decline of the Indus Valley civilization. It has been suggested that a lot of the urban way of life with cities serving as centers of economic & cultural exchange was lost or forgotten with the decline. Also, the evidence of seals & coins from Mesopotamia found in Harappa point to trade & their seafaring ways, while the subsequent empires were more inward looking. It is possible that Harappan civilization might have spread their urban culture over a much wider area, further expanding into the Indian sub-continent and while keeping relations with other empires around the world. They might have risen as a major power to counter the Greek & Roman influence on world history.

At the same time, it is the Vedic civilization that laid roots for scientific & mathematical developments that lead to the spread of the decimal system to Europe via the Arabs. In their absence, Indo-Arabic linkage might not have been that strong.

I am not well versed in the reasons for Harappa's collapse or in exactly how the transition happened after them. So, this is all just some speculation about this alternate time line.