It is sustainable if you manage it properly and don't rob the collection plate. Insurance is predicated on the same ponzi scheme, just usually private market based. Some insurance exec couldn't be embezzling millions and still have the numbers work out.

I'd say it comes down to good management (as with anything) and priorities. A bad government is going to mismanage it and spend more on the military or other priorities, borrow money to make up the deficits and then of course everyone loses faith.