Bold of you to think there will be any such thing as lasting herd immunity via natural infection, when no other human coronavirus induces such immunity.
Wrong, with SARS-CoV-1 "immunity lasted between three to six years" [1]
[1] https://web.archive.org/web/20200424144554/https://newsnetwo...
The keyword there is lasting, we know immunity exists we just don't know for how long. If a vaccine could be distributed to enough people in a short enough time frame (we're talking billions and months) it could still wipe out the virus. But if that can't be done then mandatory vaccines for travelers could work.
> ...so what is the lockdown for again?
To keep the number of infections manageable and/or eradicate it entirely, it was never about stalling time for a vaccine.