What wasn't clear about the response?
The statement "very rare for a virus to mutate to something more deadly" isn't a correct statement. In it's simplification, it's lost the original intent and as such is suggesting something incorrect instead.
A better statement would be "very rare for a virus to mutate to something that kills faster than it spreads and not burn itself out".
Those two statements are NOT the same. Even if the common perception of the second is that it means the first.
COVID has a long way to go, the ability to mutate and become more deadly, before it will get anywhere near killing people faster than it can spread. Until that time, it's ability to kill people alone is not going to slow it's spread by eliminating people it's able to spread to.
Does that help?
Edit: That's not a prediction or doom and gloom. I'm not concerned about this outcome. It's a mechanics of why a more deadly virus runs it's course. Once a virus has spread to a second (and more) host, what happens to the first host is of no concern anymore (to the virus).