To understand the projected shortfall in U.S. Social Security over the next 75 years it is best to understand the trillions of dollars involved by comparison with other tax policies and priorities. This is what Robert Greenstein of the Center on Budget and Policy Priorities does as described by Robert Naiman here.
If the tax cuts made under the Bush presidency were extended for the nextl 75 years the revenue loss would be approximately twice the shortfall in Social Security over the same period. If the tax cuts are affordable covering the projected shortfall of social security is more than affordable.
Naiman next goes on to show that the cuts projected to the military under the Budget Control Act that are now sequestered would also cover short fall to Social Security over 75 years. So Social Security's shortfall could be solved just by going through with the projected cuts to the military. It is just a question of which choices the government makes not a question of Social Security being unsustainable. Why is it not the present level of military spending that is unsustainable. For the entire argument see the full article.