Scrapping of IWT does not mean the river Indus, Jhelum and Chenab will stop flowing into Pakistan automatically or India will build a wall and stop the flow. Like this article many in India and Pakistan are thinking that way and have started accusing India of being the devil who is planning to kill 200 million Pakistanis by denying them water. These are absurd logics being put forward against the scrapping of IWT. What was happening when the IWT was not signed? What was the status before 1960? Were Pakistanis dyeing of thirst?
IWT has immensely benefitted Indian Punjab as India is able to use the waters of Beas, Ravi and Sutlej to maximum. Bhakhra Nangal Dam ensured that Sutlej is nothing more than a nulla when it enters Pakistan. Harike Barrage and Indira Gandhi Canal completely dried up the river in Pakistan and created a green land in the Thar desert in Rajasthan. Anyone can see the green line in the desert on google map or wikimapia. This would not have been possible if the IWT was not signed. India can further use more sutlej water if it released water in Sutlej Yamuna Link Canal but internal politics has ensured that Haryana remains a dryland. If IWT is scrapped and renegotiated with Pakistan perhaps we may not get as much water and Indira Gandhi canal may dry up.
India has not been able to fully utilize the 3 eastern rivers by making a massive canal irrigation system to cover the entire Punjab, Haryana and Rajasthan. If we go for further negotiations on this, Pakistan will stand to gain. However, this time negotiations wont be so easy and India can exercise the rights and advantages of being an upper riparian state.
However, by scrapping and renegotiating the IWT, the biggest gainer will be the state of J&K who have already passed resolution in the assembly to scrap the IWT. Indus, Jhelum and Chenab do not have much agricultural plain areas for irrigation. It will only help the state build more hydro power plants and store some water. This wont affect much the flow of water or perhaps 5 to 10% max. What Pakistan will lose is their "nuissance value" in delaying the Indian hydel projects.
India can always demand for renegotiation of the treaty without scrapping it as the goodwill that held the treaty is no longer there and Chinese making dams on Indus and Sutlej are affecting the river system and thereby IWT. This will put tremendous pressure on Pakistan. Another important feature of renegotiating the treaty will be involving China as this will be a tripartite treaty. As such Indus and Sutlej waters are not much of use for China but they are still making dams for strategic purposes. India can pass on the stress of holding water by china to Pakistan. The biggest lesson for Pakistan will be to not lose the goodwill or they will have to bear the stress. Renegotiating a treaty is always a possibility.