The American media seems to jump at any opportunity to dismiss China as an economic threat. It makes us feel more secure but leads to a public informed only about one side of China's story. Namely the part our ego can handle. China has regularly surpassed the US in number of cars sold since 2009. China also holds more US debt than any other country. Another way to put that is the US government owes China over 1 trill…
There are a lot of underlying facts to China's investment driven growth cycle that can be explained in math and global trade balances. Their reserves can be spent very fast. GDP growth doesn't matter so much if it's being spent on infrastructure that won't be maintained and will fall apart. In the past year or two, a giant piece of the GDP growth may have been from financial tricks as insolvent companies try to shift money around, offering high interest rates in the process (continous economic growth for a really long time mean no recession to shake out the leveraged companies which are surviving by finance alone, or outright scams.)
In the very long term, I think what will matter is the messed up demographics (shrinking population off balanced by too many males) and lack of diversity.
Japan had a similar death of growth situation after their investment driven model ran out of steam and their demographics shrunk. The big difference being their population was nearly all living a first world lifestyle by that time. China's isn't.
China can still be an important country. They can make a ton of stuff. They can wield outsized military influence globally thanks to nuclear weapons.