Physical distancing is a costly measure. It would be economically cheaper for the young and healthy to self-infect and then self-quarantine. Their chances of dying or even hospitalization is basically a rounding error in that demographic. That way, they can't spread it to other people. Herd immunity should be accelerated, not decelerated.
The evidence so far seems to show that there is immunity to it after recovery (as well as the rest of the corona viruses). Immunity is not permanent, it last 6-18 months from what I understand.
Physical distancing is another alternative but is enormously expensive to the economy.