https://en.wikipedia.org/wiki/Quality-adjusted_life_year
I understand it makes people squeamish, but the medical profession triages lives and decides treatments every day.
Doing something QALY negative is malpractice — and lockdowns are QALY negative because they harm the young to save the old/infirm.
You are going to have to show your work on that one. What numbers are you using?
You seem to be framing this that the opposite is QALY positive and we should not lockdown. Harm the old/infirm to "not harm" the young.
As other comment state, this is a "pick who suffers" exercise. Sadly, we can't save everyone. We can only do our best.
That isn't used by governments to decide on health or economic policy. Else we'd ban old people from hospitals (and save trillions in the process!) and in general make their lives a misery (high tax, low benefits) because their "value" would be approaching 0.
For example, it's used to prioritize people in transplant queues. The more "use" you can get out of a new organ, the more you'll be prioritized, but it's not the only decider.