It is more likely than not that China can succeed in turning the South China Sea into a Chinese lake over the next 10-15 years if it is determined to do so. But here is the price for that:
1. More aggressive Chinese behavior will result in much closer ties among its neighbors. The Chinese have to be worried about encirclement; this will only make it worse.
2. If the US starts withdrawing from the area, South Korea and Japan are likely to build nuclear weapons. The US military presence has brought stability to the Far East and has, in its way, facilitated the rise of China, just as the power of the British fleet assisted the rise of the US in the 19th Century. Conditions on the ground start becoming a lot less predictable, and much more dangerous, if the South Koreans and the Japanese think they are ultimately responsible for their own defense.
3. Choke points to the south and west of China present a major issue for the Chinese. It would be much easier for Japanese and South Korean ships to avoid the South China Sea than it would be for the Chinese to avoid navigating in waters controlled by potential adversaries.
If Xi were to ask for my advice, therefore, I would tell him to tread lightly.