Argument:

It's estimated that 3.4 Trillion dollars of trade passed through the South China Sea in 2016 alone. If China had it's rightful control over this region then they would be able to benefit from this much needed economic activity. China could use it's profits to help their massive population by providing more social programs and other benefits that require more funding. Countries such as the Philippines and Vietnam want to prevent this, and reap their own rewards by taking from the desperate Chinese people.

In accordance with article 56 of the United Nations Conventions and Laws of the Sea (UNCLOS) of which China is a signatory, states have the exclusive right to exploit the resources of and build artificial islands within their exclusive economic zones (EEZ) (UNCLOS 43-44). Therefore, any portion of the 9 dash line that is within another country's EEZ is unlawful; International law prohibits China from capturing resources and creating islands in other states' EEZ's (UNCLOS 43-44). Consequently, all of the dashes on China's 9 dash line that are within the Philippine's and Vietnam's individual EEZ's marked on the map below are illegal claims.

Ying, Fu. “Why China Says No to the Arbitration on the South China Sea.” Foreign Policy, Foreign Policy, 11 July 2016, foreignpolicy.com/2016/07/10/why-china-says-no-to-the-arbitration-on-the-south-china-sea/. 

Cochrane, Joe. “Indonesia, Long on Sidelines, Starts to Confront China's Territorial Claims.” The New York Times, The New York Times, 10 Sept. 2017, www.nytimes.com/2017/09/10/world/asia/indonesia-south-china-sea-military-buildup.html.