As mentioned defaults do shockingly little to change future funding. Its been years since i looked but its something like a few years of “cool down” on issuance and a few points of coupon premium. The economist has done some great, very accessible, articles on this over the years.

Second, its critical that treasury bonds are denominated in USD. The us gov controls the monetary policy and can choose to inflate away the debt over time. This is in contrast to EM debt where they get trapped with foreign denominated bonds. See also the tensions around EU debt, greece, etc.