Earlier quoted context omitted.
Do you realise that mutation is not something the species can consciously control? How could the virus have mutated to become milder to suit the social response if the society, as it stands today, is continuing to do what they previously did..?
There's likely millions of virus variants out there already. Some will evolve to become much more contagious and most won't do much of anything. When a more severe variant starts dominating and everyone goes inside and stops spreading it that makes the more severe variant die out. As it dies out, and people go outside, variants that are milder will thrive because people won't lockdown as easily. This can repeat forev…
When everybody goes inside, all the variants stop spreading. The pressure is the same on every variants, so there's no source of evolutionary advantages on any variants.