As it turns out that’s a myth. AIDS, rabies, measles, Ebola, Marburg, Hep A/B/C: none of these viruses have gotten milder, in 60+ years of evolution. There are Egyptian mummies showing signs of smallpox, a disease that killed 350,000,000 people in the 20th century, more than two millennia later.

It’s also quite obvious, mechanistically: COVID doesn’t lose many infections by eventually killing the patient who, by then, is obviously sick and most often in the hospital. It gets 80%+ of retransmission even before symptoms appear, removing all evolutionary pressure from the later stages.