CDF Forum Member "Pierrotlefeu" explains how:
WS-10C is just a placeholder for J-20 like Al-31F was for J-10. It allows the first batch of J-20s to be operationalized and put into service, which is important to get feedback for improvement and familiarize the fighter corps. The WS-15 may not appear in service until after 2026, which is actually FASTER than how WS-10 was for the J-10 (entered service 2008, indigenous engine 2019, 11 years vs potentially 8).
What is more interesting is that production of WS-10 is slated to reach 2740 engines by 2026. Assuming worst case scenario of 1/3 for spares and re-engine of older planes, that leaves around 1800 engines for new planes. Assume China will maintain the current production rate of 24 annually for J-20, which I think will be the plan until WS-15 becomes available, just like how J-10 production was kept low until J-10C became available, then that would be 7 x 24 x 2 = 336 engines for J-20, leaving 1464 engines. Assume production of Flankers and J-10s stay at the same 1:1 rate we've been seeing, then we'll have an additional 1464 : 3 ~ 488 each of Flankers and J-10s. I'm counting J-11s, 15s, 16s, and 17s as just "Flankers" here, and 2 carriers will need 2 x 48 x 2 (one regiment deployed, one spare) = 192 J-15s and 17s.
That's potentially 168 new J-20s, 488 J-10s, and 488 homemade Flankers (including naval ones). That's 1144 new fighters from the availability of a single engine type, not bad at all.
Of course, many will replace J-7s and J-8s still in inventory as well as older Flankers and J-10s, so net growth won't be too extreme for now.
Like WS-10C on J-20 instead of WS-15, it would be reasonable to expect the WS-10 series turbines to also power any early production FC-31 navalized Gen 5 fighters instead of WS-19. This would ameliorate the low-rate initial production of WS-19 and counterindicate "struggling" in that program as well.