So then how would you treat some Pakistanis claiming the heritage of IVC ?
I wouldn't treat them anyway - They are after all descendants of the same original inhabitants of these lands just as every Pashtun Tribe in Pakistan are descendants of those who lived under, fought & administered for the Ghauri, Ghaznavid, Suri, Abdali & a myriad other dynasties that the Pashtun Nation has created !
That said - We & the People of Indus Valley don't have the same culture, language or religion because cultures, languages & religions are an ever evolving phenomenon ! Even the Pashtun of today isn't cent for cent comparable to the Pashtun of a 100 years ago & he to one that was born a 1000 years ago - Culturally, Linguistically & Religiously !
The same can be said about us - If a 1000 years from now there exists No Pakistan.....this land has seen continued immigrations & emigrations, the language of those who live here has changed, they don't adhere to the same culture as we do & that the geographical boundaries of them & us are different - Would there exist some form of relationship between us twain or not ?
I would imagine that it would as there exists between you & your posterity into the extended future as there is between you & your ancestors into the extended past going to the antiquity era - Your culture, religion & languages are much different than theres just as ours are with our ancestors !
So I frankly don't see this as a thing where either Pakistan, India or Afghanistan or any other country for that matter can attach a label to whatever happened a 1000 years ago as exclusively theres or claim that somehow they are the original people while others are not for both anthropologically & historically that would be a fallacious argument !