Shifting the capital back to Karachi would not be the wisest decision. For the following reasons:
1) Islamabad is a planned city, Karachi is not.
2) Infrastructure issues.
3) Karachi is open to attack by sea.
4) Karachi has too much of a population for it to be manageable.
5) The law and order situation in Karachi is a real issue.
Among a number of other important factors.
Plus there is no reason that compels me to even consider this, why would we ? Islamabad is a model city, why would we forsake it for no particular reason ?