Just because they tend to move towards less lethal doesn’t mean they never can go the other way, as evolution is not a plan or choice. Something increases lethality but extends overall illness time, increases transmissibility, increases asymptomatic time, or a combination would outcompete other less lethal versions. While, in the long run, a less lethal virus would be more beneficial to the virus, that wouldn’t stop a more lethal version from burning through a population until it dies out by destroying its own habitat.

How long will that take? One of the excuses last summer from many Covid downplayers was the exact same sentiments and then along comes the Delta variant in October 2020.

There is no hard and fast rule that a virus will evolve to be less lethal or/and less infectious, just a noticeable downward trend in other instances. Well I'm still waiting for a downward trend even with the vaccination efforts.

What about viruses that are designed rather than evolved?

Why the downvotes? This is a really important question given just how easy it already is to design a virus.