The big problem China faced and why the one-child policy was abandoned after all is that there was a staunch focus on having a male child to "keep the bloodline alive" (cultural reasoning) which led to a stagnation in the 0-14 years age group due to an overpopulation with males. This is not fixed by killing off your elderly.

Covid was most deadly for the age group of 65 and above and in 2019, they had proportionally less people aged 65+ than the US (13.50% in China vs. 16.4% in the US). Either China's scientists failed immensely or the virus stemmed from bad hygiene practices in livestock markets selling bats.