Most Iranic speakers of modern Pakistan do not live in the Idus valley, but in the mountains and their outskirts. That is to say, territories we wouldn't consider as part of India if the British hadn't created that weird colonial border.
I would argue that the substantial majority of Iranic that do live in the valley are not necessarily recent pashto migrants from the medieval-to-modern period of domination of Iranian or Iranized dynasties, but rather the compound result of centuries of slow migration dating back to the Achemenids, with the Pashtunization efforts of the Durrani causing most of them to be linguistically assimilated (if they hadn't been already by previous migration facilitated by the Mughals).
The only way I see this as being remotely possible, is if an Iranian nation with a substantial powerbase outside of the Indus conquers it and holds it during the high to late middle ages, and makes it so that the prestige of Persian causes it to start displacing some of the local languages, which come to be seen as "uncultured". Later on, in the modern age, some form of public education is established, priorizing Persian over other languages, and discouraging their use.
And it would still achieve only partial success.