It does not really matter because what count is WS 18 analog to DK 30 and they will be mass produced soon! China has not sign contract for more DK 30 and with production of Y 20 come full tilt they soon run out of imported engine.
WS 20 is nice to have but not necessary.They can switched over when it is available
I agree. WS-20 is not indispensable, but AFAIK it is the first indigenous high-bypass engine of china. They have to show that they can create one by themselves.
Besides, WS-20 is more powerful than WS-18 and will add to the cargo capacity of Y-20.