Just a very small correction- as with all biology, natural selection will drive a virus to replicate more effectively, that’s it. This does NOT mean a virus will automatically become less lethal over time. That’s an older hypothesis that scientists found was not in line with observation.
The newer hypothesis is known as “virulence-transmission trade-off”. The oversimplification of the idea is that if a mutation increases both transmission and virulence, it will also tend to be selected for. COVID is inconsistent with both hypotheses in certain ways though, so really predicting its virulence in the short or long term has proven difficult. Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10066022/
The “old idea” is actually baked into one of the parameters of the new model. It’s why I said the old hypothesis “was not in line with observation” rather than being “wrong”. It predicted some trends correctly, but failed to predict many others. Like all science, it needed to update as we gathered more info.
The “new” hypothesis also isn’t perfectly predictive of viral evolution, but it’s more accurate with the observed spread of other diseases. Like all models, it’ll get replaced eventually by something more powerful. Likely sooner rather than later specifically because COVID put a spotlight on a lot of holes in the idea.