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.
> 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.
But viruses spread in the real world not some hypothetical lab. In the real world as viruses become deadlier they tend to provoke a much larger response from governments and populations, both in terms of prevention and vaccines/cures/treatments. That's why there's a lot more money spent on potential HIV vaccines than on vaccines for the common cold. Being deadlier is a distinct disadvantage if you're a virus trying to survive for the long haul.
So how does Omicron have any advantage, right now, from that supposed difference? Wouldn’t every mild strain just suffer from governments’ efforts targeting the bad strains, which ruin it for all the others? Note that any future benefits cannot explain evolutionary changes in the present because viruses don’t strategize.
No, deadliness doesn't really matter. What matters is how many people in infected person can infect before symptoms become too severe. Covid has a rather long period where patients are infectious while showing no symptoms at all, so it's already very good at that game.
It depends on whether you consider vaccines and human intervention as part of the 'natural' feedback process that tends to make viruses less severe.
In other words, the high-level theory is that more severe viruses either kill their hosts, or make them so sick that they stop interacting with other potential hosts. A form of social distancing, if you will.
In both cases, the virus is triggering some form of behavior change in its host that reduces its reproduction, providing evolutionary pressure towards milder viruses.
One could argue that the development and wide distribution of a vaccine is just one form of host behavior change, and should be factored in.
And the reason we pursued vaccination rigorously enough to eradicate it was because as you say it is very deadly and very contagious.
Something with a mortality rate of 30+% and kills children at orders of magnitude higher rate than COVID has is far easier to convince the world populace to vaccinate for than something with 1-2 order of magnitude lower CFR.
So you can most 100%, certainly say, it was eradicated because it is so deadly. If only say 0.5% of people died from it, disproportionately the elderly those with comorbidities, eradicating smallpox would have been a hell of a lot harder to get worldwide vaccinations on board. A hell of a lot.
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.