[scotty79]

I think this is true for very lethal viruses that have risk of killing host before spreading.

For virus that can spread before any symptoms and long after symptoms disappear and survive so long on the objects and in the air I think there will be no evolutionary pressure to get milder.

And we can get harsher version just by random chance because this virus is so infectious that it has a lot of leniency whether and when to kill the host.