Viruses survive longer when the host organism DOES NOT DIE....
New naturally occurring variants of C-19 will be less dangerous over time... just like any other virus, if the virus is extremely deadly, the epidemic burns itself out quickly.
It's not possible to make a virus that mutates to become more deadly later in a free running pandemic.
Wikipedia Marek Disease
Marek's lives in chicken dander... thus the chicken is NOT the host. The host is a product of the chicken that survives after the chicken dies.
"The first rule is not fool yourself." - Feynman