An obligation to provide an environment is not an obligation in any accounting sense.
Again, what does the existence of cash obligate the government to? A bond obligates it to pay money back. What does cash obligate them to?
You’re still missing the point. When vast quantities of $$$$ are accrued, firms, people, or countries that hold them tend to invest those $$$$ back into US bonds, since they earn some interest and are risk free place to park $$$$. This is what constitutes US public debt. As you may be piecing together (I hope you are), it's not really national debt, but rather a form of national savings or equity.
For example, let’s say you have 50K in a non-interest checking account at your local bank. You need only 5K to be available for checking, so you put 45K in a CD at the same bank. The bank debits your checking account by 45K and credits your CD/savings account by 45K. Once it hits maturity, the bank then debits your CD/savings accounts for 45K and credits your checking account for $45,098. Now have 50K and a bit more with interest. You were always in the same financial position. We wouldn’t say the bank is in debt if for 45K just because you shifted funds from checking to a CD account. When you and others constantly say the federal government is in debt because $$$$ has been exchanged for bonds it's simply inaccurate and untrue. Government bonds are basically cash, you can trade them in for US $$$$. And the government creates both ex-nihilo.
US national debt is actually a representation or saved dollars or equity. And those saved $$$$$ will never decrease in any capacity, you can basically considered them to be extinguished. Now, in order to answer your questions more clearly, government bonds never really get exchanged and spent in the net sense of things. I mean, they could, but they just sit idle and some interest is added to them now and again. This accumulation of net financial assets really doesn’t have a noticeable effect on the overall economy.