Every time the US brings up the Quad, it's basically saying "we're not strong enough to handle China on our own."
My good friend you need to correct your thinking on that. US developed this strategy of alliances so economically it works out cheaper as other nations can poll together their share of the cost. Also militarily it becomes a wolf pack and hence increases there chances many times at time of conflicts and also increases there reach to the war theatre by many fold. East works on emotions, West works on calculations and they are very effective in that.
Even US/Western economies are all in a mess and they are like driving a unstable 3 wheeler while Chinese are on a 4 wheel drive but still they are holding on to the top spot as they still have got plenty in reserve. Don't underestimate the alliance of thugs and that will be the foolish thing to do to write them off. Lets hope China develops peacefully and decline of the US/West be part of a natural evolution like the past world powers before them.