China drew the line back in early 1900s by ROC government.
Nobody criticized it until recent decades.
On the contrary, I don’t think China would launch a large scale military action in SCS now, because it is contrary to its current own interest: China would do anything to
maintain the momentum of its development. Any war would interrupt the momentum.
Once China has more money, and grows more muscle, and is therefore more confident, more options would be on the table, including but not limited to subvert the existing governments with little or no violence.
Even China resorts to forces, it MUST promise an open sea lane for every (non-hostile) country to gain support and to benefit itself. Closed SCS sea lane is not in China’s economic interest.
Small nations need to utilize a big power to balance against another power. This is international relationship class 101. Vietnam War is a typical example. If the small nations do not play the fire carefully, the nations themselves run the risk of vanishing from the world map.
Vietnam (and the Philippines to less extent) can change its friends, but it can’t change the fact that China is its neighbor.