This could be the "endemic version" of COVID that we've been looking for. Particularly if it really is so much more infectious than the other variants and so manages to displace them.
But there's no particular reason COVID would need to get any more mild than Delta.
Can you elaborate?
Covid has a long incubation period before symptom onset (when somebody notices and isolates or becomes bed-bound). So Covid, especially Delta, spreads well for days, whether or not ultimately symptoms are mild or ultimately it causes death.
Therefore a mild version of Delta wouldn't necessarily outcompete a non-mild version of Delta, all other things being equal.
This need not be true. There are many ways a virus could spread more while simultaneously making people sicker and causing more death. For example, a strain evolving to cause people to be contagious and spread for twice as long as now and then killing everyone could very well spread more than it does now.
Example: common cold viruses. They're essentially always there and only make you sick when your immune system weakens a bit, and goes away quickly as well.
However, because we have vaccines, I do think ultimately you're right. If it turns out that omicron (or another variant) is basically a cold but also evades current vaccines, I think society will 'allow' it to become endemic.
In this sense, low vaccination rates of Africa may be just what we needed.
Is there any evidence of diseases that have selected to be less deadly, aside from by killing all of its especially vulnerable hosts? As far as I can tell, this is naive optimism: the selection pressure for not killing your host is irrelevant if they're only going to be infected for a week or two anyway, and basically every mutation that's going to increase transmissibility is likely to be negative (increased viral load in infectious agents, immune escape, etc.).
It sounds like you're asking for specific examples of pandemics that didn't happen because they weren't detrimental enough to endanger people. That category of respiratory viruses is often broadly described as "the common cold."
It probably was endemic, as it was declared eradicated in 1979 by an effective vaccination campaign.
More importantly, though, smallpox, as a complex DNA virus, had far fewer viable mutations. That fact, along with an apparent lack of non-human reservoirs, is probably why it could be eradicated.
https://www.gavi.org/vaccineswork/will-covid-19-evolve-be-mo...
HIV is clinically a mild virus and doesn't cause any severe symptoms by itself. It is only when HIV becomes AIDS that it is dangerous, but then again, the symptoms are not caused by HIV/AIDS itself, but by the lack of immune cells caused by AIDS. You could have HIV forever and not even notice it if you were in a vacuum as you say :)
We can't know how severe Omicron will be, but let's hope it just becomes a cold. As OP said, the most enduring parasites (viruses are categorically parasites), are the ones that don't kill their hosts. This is not only true for viruses, but all kinds of parasites.
It could happen regardless of external factors, so I guess my "vacuum" analogy wasn't perfect. All I mean is that HIV/AIDS itself doesn't kill you by itself.
In the highly vaccinated version of the story, vaccinated people are mostly unaffected by the severe effects, so they go about their lives and, if they happen to be the ones with the harmful mutations, they give those variants a fighting chance to spread. It would be different if the vaccines reduced viral load, but the limited studies available don't seem to point in that direction.