I'm not pretending coronavirus is literally a type of flu virus. It just happens to be a novel flu virus that we don't have as much exposure and immunity to yet. There are plenty of historical examples of what happens when a population is hit by a virus that it has little or no immunity against, even though that virus is relatively harmless to those with immunity.
That is not an argument against vaccines, and it is not an argument against all the precautions that were taken when Covid-19 first hit. Those were both necessary for the population to build as much immunity as possible, with as few as possible deaths and as little as possible sickness.
It is an argument for the fact that Covid-19 must be treated differently now and in the future vs. how it was initially treated. It is now a virus that most of the population as some degree of immunity against (due to both infections and vaccines). If you doubt that that's the case, just look at the reproduction numbers for Covid-19 outbreaks, which are still ongoing. In the initial waves, just a handfull of infections were capable of spreading to entire countries, killing thousands, within just weeks. If a handfull of people get Covid-19 now, that is no longer the case, even though we aren't quarantining people. This is a direct result of herd immunity. Just like we have flu season, where different flu viruses spread in local epidemics, Covid-19 will continue to spread in local, seasonal epidemics in the foreseeable future (likely "forever"), but it is no longer the same threat as it was when nobody had any immunity to it.