The Chinese govern the production hardware. If they suddenly stopped producing miner, it would take a long time to grow back to a network of more than 1 Exahash. Without growth of computing power, the network becomes more vulnerable to the consolidated operations of large computing power.
Regardless of the results of the consequences for bitcoin will be dramatic and irreversible. If Americans and Europeans will not be able to seize the initiative and break the Chinese domination, the future for bitcoin can be grim.
I also advise you to see a list of the 21 companies that control bitcoin: http://www.businessinsider.com/bitcoin-pools-miners-ranked-2015-7
Yes, once the Chinese decide to drop bitcoin and support another one then it could be game-over but it won't happen in the near future yet since the Chinese are heavily invested the hardware miners unless they're already selling them little by little to outside their country via ebay or other online merchants.
Not only are they heavily invested in mining gear, the biggest mining farms have been stashing more or less 1,000,000BTC throughout the years. They will most likely never jump into an altcoin.