I think people have to understand the origin of the dollar reserves before imagining the implication. In the cases of Chinese and Japan (and most other export-oriented economies), the accumulations of dollar reserves are the by-product of the exchange rate manipulations, which are to make their currencies undervalued to maintain the export competitiveness.

The US dollar has been the 'target' of such exchange rate manipulation is mostly because of its role as the largest import economy. As a result, if China becomes the next big importer, other export-oriented countries will probably do the same trick like it has been doing to the US.