From what I've read over the years, the "good" news about most virus mutations is that they tend to mutate towards survival and not necessarily lethality. That was one of the observations about other viruses like Ebola etc. If the virus kills off the host too quickly or efficiently, it won't last long, or propagate. So a mutation towards increased transmissibility makes sense as being more probable as a real world concern. There likely are mutations that are more lethal out there but tend to end up being self-limiting...unless host behavior encourages ease of transmission (looking at the mask Refuseniks here), I'd imagine...