Refugee crisis is indeed a deep issue with any nation, since it's around 3 decades now, it will be almost impossible to send back Afghanistan refugees and illegal immigrants back to their motherland since they would have quite intermingled and entangled within Pakistani society.
I am saying this after having first hand experience seeing East-Pakistan/Bangladesh refugees and some illegal immigrants in and around Kolkata and West Bengal. Even after Bangladesh became an independent nation in 1971, a large group of refugees choose to slip in and remain within India and till date we couldn't do anything about them. Given this scenario, it will be a herculean task if ever Pakistan want to send back majority of Afghanistan refugees.