It's about lowering the threshold and the R0 factor. If the virus burns through, say, 20 percent of the pop, but if the remaining 80 percent wearing masks, stay within their social circles, avoid crowded spaces, and social distance, the virus will find fewer and fewer people to infect and die out. Sure, it can be reintroduced by an outsider, but a comprehensive testing and tracing program can nip that in the bud. This happened with SARS-1. You didn't need herd immunity. It just died out because it had trouble finding new hosts due to the mitigation efforts in those countries (SARS-1 is what prompted Asian countries to wear masks all the time outside).
No eternal lockdown. There's much progress being made on a vaccine.