They are using ws10 as a stop gap measure as china want to continue with local options.
The most common problem with WS10 is very small MTBO and MTBF is very low. The metallergy of the blades is a problem due to which it requires extensive maintenance.
Furthermore ws10 starts installing only after 2011 and china places order of su35 at the same time
Short MTBF is just the initial batch and the continue media spinning of past old stories. WS-10 MTBO is 6000hrs and much higher than AL-31F. There is no proof of short MTBO and exist as a problem now. Why would J-10C and J-20 switch to domestic engine if WS-10 still a problem?
Su-35 of 24 purchased is just a the minimal requirement to trigger a sale. If so good, why stop at 24?