Well, you have to really believe in the Fed in order to think that the dollar will not collapse. By keeping their reserves, they risk losing much more.
That will prove true, eventually. In the meantime, 'normalcy bias' limits the logical conclusions here.
Why?
The Chinese improve has not so much to do with the US buying their stuff with dollars but much more the Chinese being able of producing these stuff in the first place. They should stop accepting subsidizing the dollar and start feeding their own internal market with their production. It's true that their internal market demand is probably not the same of that of US and therefore the capital structure would have to change, what can be a bit painful. But it would be in their own benefit. They should really stop being mercantilists and let the Chinese people enjoy more the increase in capital the country has gone through.