As long as they want to keep the dollar value high and their own currency cheap, so that we continue to buy more Chinese exports and thus employ more Chinese people. And they have a lot of people to employ.
They could let the dollar gain value, cause Chinese exports to fall, and cause US exports to increase, creating more US jobs.
There will always be buyers of US treasury bonds. They are sold at auction, where the bids are interest rate offers. And the interest rate that people are willing to accept is lower than what it was 10 years ago.

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