"It depends" is usually a safe (if not always helpful) answer. The older idea was that viruses always evolve to be less virulent/deadly, but that didn't match observations. The newer idea is simply that of a virulence-transmission trade-off. But there are various assumptions behind both ideas, and not all viruses follow those assumptions, so.. it depends. A decent review: https://pmc.ncbi.nlm.nih.gov/articles/PMC10066022/