A silly notion that raises its head like clockwork. Foreign bondholders account for roughly a quarter of US debt. The top two countries are Japan and the UK, followed by China. Two close allies lead the list - collectively they hold 3X what China does.
US treasuries are favored because the market is extremely large, liquid, dollar-denominated, and widely usable as collateral. There is no single alternative market with comparable scale and liquidity.
This will not happen, even if foreign bondholder somehow launched a coordinated effort, which they won't.
None of this will stop people from continuing to roll out this tired, retread notion.