That's only for viruses that rapidly kill you. Even the ones in childhood vaccine schedules kill you too slowly to become less deadly over time, and they kill people way more quickly than COVID--which takes almost 3 weeks to kill people, and sometimes only after a repeated infection

The mass exposures in China could very easily result in a variant that is both more deadly and more transmissible. Hell, all it would need to outcompete Omicron while being deadlier is a longer latency between infection and symptom onset