Earlier quoted context omitted.
> But there's no particular reason COVID would need to get any more mild than Delta. Can you elaborate?
Viruses evolve to become more mild because very sick or dead hosts are bad spreaders. Covid has a long incubation period before symptom onset (when somebody notices and isolates or becomes bed-bound). So Covid, especially Delta, spreads well for days, whether or not ultimately symptoms are mild or ultimately it causes death. Therefore a mild version of Delta wouldn't necessarily outcompete a non-mild version of Delta…
This need not be true. There are many ways a virus could spread more while simultaneously making people sicker and causing more death. For example, a strain evolving to cause people to be contagious and spread for twice as long as now and then killing everyone could very well spread more than it does now.