The evolutionary pressure you're talking about isn't a given. The Influenza causing the Spanish flu became deadlier in the second wave, it then evolved to be m

The evolutionary pressure you're talking about isn't a given. The Influenza causing the Spanish flu became deadlier in the second wave, it then evolved to be milder in the third wave, however, still more dangerous than in the first wave. The evolutionary pressure is for reproduction, and if other factors facilitate reproduction, like a large window during which the infected are asymptomatic but contagious, or maybe airborne transmission, then the virus can very well be deadly. It appears SARS-Cov-2 is very well adapted given how contagious it is. Personally I fear a more dangerous second wave in autumn or next year.