Yes, if America were to finally "pacify" Iraq and Afghanistan, reduce Iran, etc, etc, then it would "zoom in" on China (and of course, Latin America and a few others). But my feeling has been for a while that the DPRK is a different story. To me there is something surreal about all this "concern" expressed by the US over North Korea. Did they forget that both China and Russia, right next door, also have nukes??? Why are they so hot on this small problem, when China and Russia, besides, also have navies, ICBM's, nuclear subs, and real economies to boot?
I think American policy towards the DPRK is aimed at creating a problem, not solving it. The reason is that no matter how big the problem, it is not actually an American problem, but rather, a Japanese and South Korean one. The US is only trying to paint itself as the "defender" here. Because China has no choice but to protect the DPRK, when the US stirs trouble here, it is adding one more bone of contention between these two countries and China.
Yes, American military forces in the region are aimed squarely at China. Even the Korean War itself was aimed squarely at China (and the Soviet Union). But for South Korea and Japan there has always been a second, if smaller, reason: the DPRK. And this is something the US exploits.
China's policy in this region is the same as everywhere else: to grow 10% every year. If its economy is roughly equal to Japan's today, it will be 3 times bigger than it in a dozen years. At that point, I don't think Japan will have anything over China technologically speaking either. Under these conditions, when you also consider that Japan already exports more to China than to the US today, that Chinese influence has already surpassed American and Japanese influence in Southeast Asia (important for Japan economically), and that Chinese military power will soon be the strongest in the South China Sea (important for Japan strategically), it simply makes no sense at all for Japan to be allied to the US. Such an alliance would only make Japan a target, without contributing to its defense. It would become a liability, after having served as an asset for them for so many years. In a decade or a little more, Japan will be in a similar situation as Vietnam is today, with respect to China.
But if the DPRK situation remains the same, Japan and S.K. will still have this added reason to rely on American military involvement. The US is already relying on this added reason, the DPRK'S perceived belligerence, and now also their nukes, which are important for public opinion in Japan and South Korea.
It is in this context that I see the DPRK as a bit of a headache for China. For all the pieces to fall into place, Japan has to feel comfortable about the two Koreas, which it has mistreated as badly as it mistreated China. Perhaps not now, but in the medium and long term, it is up to China to arrange a peace between the DPRK and its two other neighbors. Of course, this cannot mean sanctions, especially today. And it is not a matter of "cooperating" with the US. But I do think that the attitude of the DPRK is a problem for China, because 10 or 12 years is not that long!