And if US use " sea sanction " blockading SCS and China did not have any preparation, almost all China line of trade via sea will be cut.
What happens if the US blockades the Malacca Strait? China would be in just as bad a position. Does this mean we can expect China to start occuping islands off the coast of Malaysia next?
Never think "weak China " will be spared by the west. Remember what they did to Qing ?
Making modern defence plans on the basis of events more than a century ago is pretty boneheaded. Everyone's been attacked by someone in the past, so should we all be assuming our neighbours are going to attack in the future? China isn't the only country that has ships going through that area. Putting aside ASEAN states, Taiwan, Korea and Japan are also reliant on those shipping lanes. Would China like it if they established some military bases on neighbouring islets? No, it wouldn't. But they'd only be following the same principles as Beijing.
Also, you're conveniently forgetting that for the past 30 years the US and the rest of the developed world have been investing heavily in China, and that doesn't even account for the Cold War era of seeking good relations with Beijing. Why on earth would the US and others wait until China had got to this stage of modernising its military to attack? They wouldn't, they would have sought to blockade China in 1990 or 2000.
What you're really hinting at is that in the future China might take aggressive action against some of its neighbours that might lead to a US-led group of countries putting economic pressure on Beijing, not just via sanctions but also maybe by stopping its ships in international waters. That would be a threat to China but also due to a situation of its own making.