The trouble is that how easily a virus transmits from one host to another doesn’t necessarily correlate with how sick it makes its host. Take the bacterium that causes
cholera for example, which needs people to get sick – often deadly sick – before it can spread, principally through diarrhoea. Respiratory viruses need people to be close enough to be breathing the same air, to be transmitted to a new host, but don’t necessarily need them to be sick. SARS-CoV-2 is thought to be
most infectious shortly before and during the first few days after people develop symptoms. The most severe COVID-19 symptoms don’t tend to develop until the second week of the infection, by which time most of the active virus has been neutralised by the body’s immune response. Indeed, the catastrophic organ failure and breathing difficulties experienced by people with severe COVID-19, are largely driven by an
overactive immune response to the virus, rather than by the virus attempting to transmit itself to a new host.
So, unless SARS-CoV-2 becomes so virulent that it causes people to become severely ill and self-isolate before they transmit the virus to other people, there is no pressure on it to become less deadly.