http://sports.sina.com.cn/others/badmin/2018-06-15/doc-ihcyszrz8517588.shtml (I translated only the parts on Xie Zhihua and Cai Yun's takes on the plight of CHN WS and their Uber Cup'18 downfall)
Xie Zhihua (the China-born Thai coach of Intanon Ratchanok) originally guessed that He Bingjiao would play WS2 (in the semifinal match with Thailand), "because He Bingjiao has a superior record against Thai players." The author specially looked up the relevant H2H records - HBJ vs Nichaon Jindapol is 2 - 0, undefeated, both matches played in 2016; even if it's versus Busanan Ongbamrungphan, HBJ's record is clearly advantageous, 4 - 1, winning the last four encounters.
As to why He Bingjiao was not fielded in that tie, CHN's Singles Head Coach, Xia Xuanze, had already given his explanation:" Earlier in the preparation stage, indeed He Bingjiao was selected as the WS2, but during training there was a little mishap, she injured her ankle. After a period of therapy, she recovered fairly well. In the preliminary rounds, we tried her out, let her run about to see how it goes, as her training was disrupted for more than 10 days, but from her performance in the preliminaries, we could tell there was still some problems with her leg."
However, Xie Zhihua has his own thoughts on this matter, he candidly expressed to Sina Sports:" Actually I felt that they could still let He Bingjiao play WS2 and have Gao Fangjie as WS3. Also I have no idea of Li Xuerui's extent of recovery but from that match she played (against Busanan), I think she hadn't recovered well, her movement was much slower than before. Based on our expert observations, we can tell she looked apprehensive while running about. For such an important match, we need to have a physically and mentally fit player, I felt that Li Xuerui should not be fielded that day. Li Xuerui gave me the impression that the will is there, but not the strength (the spirit is willing but the flesh is weak), she had misgivings while running about, I could tell from the match that her mobility on court was no good."
Now, why do CHN Women's Singles fall to such a state ? Xie Zhihua voiced his personal views: " My coaches have also said, it's because they did not attach importance to nurturing the junior players previously, not meticulous enough in tactics and techniques training. Now a 'gap' has arisen, this time many players are appearing for the first time (in such a major event)."
When asked "why have CHN WS fallen into such a predicament", Cai Yun also gave his own views:" After all, I am not inside the system/organization, my understanding may not be comprehensive, but Women's Singles as far as their talent-spotting and selection is concerned has indeed been facing a challenging situation. Some coaches have told me, nowadays the number of players for selection to turn professional is small, in addition, the training regime for singles is boring, arduous, the talent pool has actually diminished."
How do we change the situation as quickly as possible? Facing this question from Sina Sports, Cai Yun's response was " strengthen player's adaptability and ability to resolve problems, increase shot variation, have a wider repertoire of strokes. Nowadays women's singles play at an increasing pace, therefore the technique to control the fast rallies has to improve. Furthermore, young players must not lose their vigour and dynamism! "