Because A.) Approx 1 20yo in a million will have life threatening complications. Plus B.) there are a lot more of them than their (are people who have non-negligible chance of dying + people around those with non-negligible chance of dying).
The lockdown is justified by certain measurable factors, such as hospitalization rates, the risk of death, and the risk of major complications, and as we vaccinate the population, these factors will decrease. They will decrease quickly at first and then very slowly taper off to zero. (Remember there will be idiots who avoid the vaccine altogether.) It makes sense to ask when Covid crosses the line from a risk that justifies lockdown to a risk that does not.
