There are lots of F18 out here. Potentially there could be over 300 of F18 gathered at one time at SCS. Probably no way to match that 300, but how about match half of that which is 150. Right now China has about 24 j15s. World wide US has close to 1000 F18s. Think about it for a second...
I don't think China can produce nearly enough j15 therefore cheaper and faster produced navalized planes needed.
China has other land base aircrafts they would use if an opponent came knocking at the door of China. Currently the 2 stobar carriers can carry 60 aircrafts combine. 24 old j-15 + 20 new j-15 = 44 aircrafts by sometime 2021.