I would say the the US is still THE superpower in terms of miltary and economically. The idea that the US is on the verge of collapse as some like to claim is no more valid that than the claim that China is on the verge of collapse. The US has been THE superpower since the end of WWII, but we are seeing an ever increasing rise in economic prosperity around the world [but of course with some regions not advancing as much and few countries sliding back].
China used to be the sole world superpower, and the Chinese would like to return to that status. But the Chinese are presently a significant power- both economically and miltarily, and will undoubtively continue to grow in power/influence. I don't think China has a chance of equalling or surpassing the US as a superpower unless somehow demoracy/free market/free press is somehow significantly advanced in China. I think China's only hope to become a superpower is dependent on using the knowledge base associated with Hong Kong- the people of Hong Kong at least have a vague clue of how to bring this about. So, if the "collapse" is in terms of becoming a superpower, I agree that this is likely, but I expect steady progress and of course they will be setbacks- but probably not worse than what Japan is going through & and has gone through.
India generally speaking has a different set of problems than China, but I think there is a better chance that India could become the superpower in Asia.
As far as the China starting some war. I think the Chinese leadership will not be willing to do this- it could weaken their support and lead to destablizing China. Though if some nation is foolish enough to attack China, it would probably unite the people and strengthen the leadership. Miltarily attacking Tiawan would be a very bad mistake, and I don't think the Chinese are that stupid. There is absolutely no way the China could successful invade Tiawan. It simply won't work politically- even disregarding that fact that the US would demolish any sustained miltary assault on the island.