>what are they going to buy annually that will make a difference at this point

Mechanically, "they" being sovereign banks serve as price-insensitive marginal buyers that close treasury auctions regardless of price because they buy treasury for storage/liquidity. VS domestic buyers (hedgefund insurance), who are price sensitive = raise rates to attract discriminate buyers who buy for yield/valuation = worse debt servicing = faster debasing. Foreign sovereign buyers still play governor role in making sure domestic buyers get a shit none-market deal, i.e. US gov gets a good deal which moderates velocity of debasing. Of course past certain level of debt brrrting, the ability for sovereign buyers to absorb is compromised, in which case it makes sense for US to fuck foreign buyers over and inflate away as much debt as possible while still reserve currency - US can inflate/soft default faster than world can unwind. And TBH US will probably be "fine" as long as US can still gunboat diplomacy. If can't be banker anymore, be the mob boss.