> viruses tend to become less lethal and more contagious

This is not always true. Remember, there has to be some sort of evolutionary selection for it. As an example, the reason delta even became a thing, which was more deadly than the original strain, is because covid was spreading before people had symptoms. So it didn't matter if it killed the host or not, because they likely spread it before they even knew they had it. So there was no selective pressure for it to become less deadly. If covid had only spread after someone was symptomatic, it may not have turned into a pandemic at all

We just got lucky that the omicron mutation happened. If delta was still circulating, we would be in a very bad spot