The most possible scenario is that we might see economic crisis. But most likely we won't see dollar collapse during our lifetime.
It would be worse than Zombie Apocalypse and US Gov knows that. They have instruments to prevent that situation and they won't hesitate to use them.

And even with totally unplayable debt economy of US could exists because of still strong productivity.
This however can change quickly as we now observing productivity crisis -  'US productivity slips for first time in three decades'.

Interesting read: http://www.ft.com/cms/s/0/925d8e6c-226f-11e6-9d4d-c11776a5124d.html#axzz4Dddo5sGS