If Pakistan falls to powers openly hostile to the US and if the US attempts to relieve Pakistan of its nukes, the consequences will be profound.  Successful or not, the frustration and anger of the Islamic world would overflow.  
Overwhelming Afghanistan and invading Iraq are hardly comparable to engaging in an act of war with a country of a billion people.
I won't offer a prediction for the future, but we should not expect some fly-by-night nuke rescue to solve the over-arching problem.  Afghanistan is not going to remain in US control for long, that's for sure.  Guessing what happens next does not include any happy endings.