No matter how much US gets a debt downgrade, it will still be able to borrow money. It can get a bond rating of Quadruple F- and there will still be countries who will be willing to lend to the US. Who else are you go lend the money to? Russia? China?? Both countries with autocratic regimes? You are even worried about the USA defaulting on its debt so you would rather lend money to other countries that have even weaker economy and less stable political climate? LOL
No the USA borrowing money is like Trump going into debt, the more money that USA borrows and seemingly has trouble paying, the more money will be lent to it because that would be the only way to ensure that the USA would be able to repay the debt. LOL Trump was at one point in arrears for his debt for $640 million, what did all the banks that he owed money to do? They gave him more money so he could keep paying back his debt. Same thing will happen to USA.
This debt-ceiling talk is just a show, a show of good-faith on US's part, nothing more.