The working population within China is falling, this causes rises in their wages altering their ability to undercut other countries exports on low wages alone. This might mean lower treasury debt holdings. Naturally secondary developments and consequences occur, I think these are the main reasons why dollar would fall. Ultimately politics either foresees this change or fails under the strain of the inevitable
The retirement age for Chinese man is 60 and for woman is 55. It will be raised to 65 and old in the near future. So the working population will not decrease too much.