Those who are already here arrived illegally and by giving them citizenship you are telling other's that they too can come illegally and wait their turn to get citizenship.
Why is that too hard to comprehend?
Or we can make it impossible for them to have a living and thus they will leave at their own choice through the same way they came illegally.
And if your concern is crime then that too can be easily solved by improving security and law enforcement through reduction of bureaucratic corruption.
Pakistan-Afghanistan border is being sealed and wont be porous as it was before. We have greater control over who enters and who exits the country via the border now. Furthermore, issuing and registration of refugees via any form of identity will allow to us find newly illegally immigrated people easily and deport them or send them back, so it sort of helps.
Forcing them to leave will result in increase in crime, increase in recruitment by terrorists, increased terrorism as a result, increase in violence and increase in death and destruction. Not even the most modern and civilized civilian security would be able to handle this without violence and in a timely manner. It's practically not possible to play the 'improved security will handle it' card because it will take at least a decade for us to improve our civilian security forces to the standard where it is above satisfactory.
And it will take decades for us to make all of them leave. So, why make them leave? You make them leave, what do you get in return? Years of time gone, money gone, infrastructure damaged, terrorist narratives promoted, anti Pakistan sentiment built up. Why not make the best out of it?