well indo-gangetic plains have almost always been the same bunch of people. so there is no question that most of the arable parts of pakistan were carved out of what is geographically the north indian continuum. there is no natural border between india and pakistan. besides, if i remember right, pakistan was made for muslims of india.
if you want to talk separate nations, the baloch and the pashtuns are iranic people, punjabis and sindhis are indic people. there is a natural geographic isolation between these populations. looks, culture, language - vastly different.
India was a region without doubt, and Pakistan was made for the Muslims of that region.
The issue is whether a unified Indian country ever existed. The nearest was the Mauryan under Ashoka and later the Mughals, but neither of these empires did not include the southern extremities of modern-day India.