I don't see why India can't occupy PAkistan. There are parts of Sindh especially which would very much prefer Indian occupation as they are sore of having controlled by Punjabis (most of the military and political establishment in Pakistan are Punjabi muslims). Any future war will definitely see India reoccupying all of Kashmir and other strategic areas so as to deter any chance of future Pakistani infiltration. It will mean that we will push Pakistan back to the plains.
India could occupy parts of Pakistan, but there are simply far too many Pakistani civilians who would actively resist a full-scale occupation.
India could probably defeat the Pakistani armed forces (although one wonders at what cost given the nuclear capability of both countries), but they couldn't really occupy and subdue the civilian population which is heavily armed.
Hell, Pakistan hasn't even conquered its western provinces.