Even if the economy were growing at a faster pace, it wouldn't come close to offsetting the interest payments on our ever-expanding debt.
If you want to know why the Status Quo is unsustainable, just look at interest and debt. These are not difficult to understand: debt is a loan that must be paid back or discharged/written off and the loss absorbed by the lender. Interest is paid on the debt to compensate the owner of the money for the risk of loaning it to a borrower.
It's easy to see what's happening with debt and the real economy (as measured by GDP, gross domestic product): debt is skyrocketing while real growth is stagnant. Put another way--we have to create a ton of debt to get a pound of growth.
There is no other way to interpret this chart.