A default doesn't mean a national collapse. Argentina has defaulted multiple times, they're still getting by just fine.
The problem will come in the form of imports, international assets, and foreign financial aid. These would disappear over night. Local industries would be affected, and many shut down until alternate sources can be found.
A nation like Pakistan won't collapse, but it will definitely become poorer.
From a military view point: I don't know if the army leadership knows just how bad it will get for them, because all those shiny toys that they've been buying, they're gonna end up being glorified door stoppers. They won't have the money or supplies to maintain them thus aggression from Afghanistan and India will increase as a result of all this because they will 100% smell weakness from Pakistan.
