Those foreign nations take USD out of circulation and buy US treasuries. They are expressing a desire that the US invests the USD they spend on the bonds. When you buy US treasures you do so because you think the US government can spend your money better than you can spend it yourself. If the US decides to not borrow your money it is basically saying you should put your money elsewhere but these foreign nations refuse to do so.

Because the US government doesn't spend the money inside the US the obvious result is unemployment. There is no way to increase the investment rate to account for the increased savings rate. Instead, the domestic savings rate of US citizens must go down to compensate for the increased foreign savings rate. How does it go down? Someone who is unemployed or underemployed must consume more than they earn. Income inequality becomes mandatory to just keep the economy alive. It's increasingly unsustainable for everyone including foreign owners of treasuries.

Of course the US has a nice president right now that wants to let the damn savers save by increasing the investment rate through infrastructure spending. It's a win-win for everyone. The underemployed get jobs, the savers get a nice investment that will not suddenly collapse one day.