Secondly when these "ex-leaders" were doing bad things, many in the "current-leadership" including bureaucrats and generals were their helpers. So the evidence of any wrong doings evaporates in the thin air.
And court system is utterly dependent on "paper trail" or evidence. In the absence of evidence, courts have to let them go. The usual result is that the top guys go home and the nation is left with so much bitterness.
Depends on how you look at it. Zardari can be brought to justice to show how he accumulated the wealth, Nawaz Sharif should've been made to pay off the loans he got forgiven (sell his properties to pay off the bad loans). It depends no how you pursue the case. If all you want is jail your opponent and use 100 200 year old justice system to show "paper proof" then yeah what you said would happen.
Thus it is not to let anyone go "scott free", but rather to reduce the bitterness and intolerance in the nation, the top leadership should be allowed to live off their last years in peace.
Looking at the results of the recent past "Nabbing" and prosecutions, it will be reasonable to say that we should not have made the case against Zardari, or Nawaz Sharif either.
All that corruption hoopla went to waste when the nation elected / selected them in an open election. So for ordinary people it didn't matter if AZ or NS did anything wrong. We the educated elite should learn from these "ordinary people" and quit arguing about innocence or corruption of the top leadership.
Until "bitterness" is addressed by the whole (governance) system Pakistan will continue its downward spiral. If we can't do that then there is no point in having justice system at all, this is where mafias/dakus/criminals take rise and provide this as their "justification".