Virus evolution isn't the same as say human evolution.
The finches that Darwin observed got their big beaks because they improved survival chances - survival of the fittest to reproduce improves the breed, crudely, although there might be the occasional dead end and extinction through over-specialisation. Mutation has an essential role in evolution and every useful feature will have originated with it but breeding selects for the useful ones.
AIUI, viruses don't come in male and female form to selectively combine useful genes so depend solely on accidental mutation plus infecting hosts to change. A mutation can be useful, or the opposite of that. I'd guess most are probably for the worse (for the virus). It might not be sensible for a virus to evolve to kill its hosts but that's not going to stop it. Such a mutant might die out quickly, but that's not guaranteed, especially if it happens to be nicely infectious and transmissible for a reasonable period before the host succumbs.
i'm aware that evolution is actually very complicated, I'm no expert and I have quite possibly misconstrued it but I'm pretty sure it's wrong to assume that Covid could never become much more lethal even if there does seem to be some inverse correlation between severity and infectiousness.
The downside of Omicron is that there is a huge amount of replication going on with the high rate of infections and therefore more mutation opportunities.
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