Indus valley is tamil Civilization

[Kabira]

This is what happened to IVC, people simply went back to 'primitive-life' and grew back to Kingdoms. Many of these Kingdoms like Gandhara began to take form even before IVC finally 'collapsed'. What we saw shortly after were emergence of Kingdoms like Sindhu, Sauvira, Sagala, Gandhara, Taxila, Kapisthala, Bahika, Madra. Did they randomly poof into existence? nope, they were the continuation of the IVC remnants.

Remember proto"indo-aryans" were very primitive beings. They were good at warfare because they used to fight and kill each other over limited resources to live in harsh place like steppes and before likely further north. But because of their dominance in warfare they could impose themselves on Harappans. All these indo-aryan civilizations like gandhara no doubt were thanks to Harappan people just that their language couldn't survive.