>that means that LOCKDOWN_COVID19_MORTALITY_REDUCTION - LOCKDOWN_EXTERNALITIES >= 20 million wellbeing-years.

Absolutely wrong. And you are missing:

- lockdown reduction in suffering, due both to the acute phase of the disease and after effects

- No-lockdown-externalities, including damage to the economy if the disease is allowed to spread through the entire population

And perhaps most importantly:

- the risk that a huge reservoir of infected people will lead to repeat waves caused by mutations, and even worse that cross-species infection occurs (we already know cats and dogs can be infected - pigs and poultry could be next); then we risk flu-like antigenic shifts.

Put all this into the equation and I think the only thing we should be contemplating is how to eradicate the virus as soon as possible.