What make you think it will stay soft? It will be the last nail in the coffin of Pakistan if Gov of Pakistan go one step back from their current stand regarding Durand Line !
I don't that you have to worry about the border being an issue. The majority of Pakhtuns have always been patriotic Pakistanis and will remain that way. What we need to do is stabilize Afghanistan: kick out India and work with Iran. After that we can open up travel, trade links across the soft border. Soft border does not mean the end of Pakistan sovereignty in the area but instead the reduction of controls.
Besides with the Americans regularly doing cross border attacks, where is the Pakistani state?