You are 100% correct. Now please go through 1965 and 2010 Afg. Pakistan transit agreement.
Both states, Afg. has a right to use Pakistani transit route about Paksitan doesn't have. Now Pakistan needs a transit route from Afg. to central Asia for CPEC. So it is Pakistan who wants a re-negotiations for the transit agreement. Afganistan states, Ok if you want a re-negotiations currently our trucks go until wagha border carrying goods to India, and come back empty or with Pakistani goods. Afgs want that their trucks should be allowed to bring Indian goods to Afg. from Wagha border.
So, Pakistan doesn't want to cancel the agreement because of the prevailing relations between the concerned states. As India and Pakistan relations are in a much worst situation than Afg.Pak relations and it will give a free hand to India to cancel IWT for the same reasons.