If we don’t vaccinate a critical mass, the vaccine will lose effectiveness as the virus will continue to spread and mutate unmitigated among the rest of the population.
In 1919, you either lived or died, and that was it. If the virus killed its host, that was bad for the virus, if the person lived, their immune system defeated the virus. We now have people living on vents with live virus mutating in their compromised immune system for months. Our improved medicine is actually giving the virus an advantage in that sense; however, we have the vaccine to combat this, so that people stop getting sick and put on vents (providing opportunity for rapid, wild mutations), and the virus runs out of viable hosts...we should use aggressively.