A Republic of China foothold on Hainan

How would the South China Sea situation change if the Republic of China retained Hainan?

It would gain strategic depth, but Hainan would be exposed and require continuing defence.

Retaining Hainan would provide strategic depth and another defensive position. The island would also remain exposed. An opponent could concentrate attention on it, requiring continuing concern for its defence. The counterfactual creates both additional space and a persistent vulnerability.