the problem here is, the death rates of both diseases dwarfed that of COVID, and as such, that makes the diseases far deadlier (having similar contraction numbers). The deadlier a disease is, the more difficult it is to reach herd immunity. With a relatively small number, in this case, six million, we could cut off the number almost immediately for shooting for herd immunity. It would cause a spike in cases and deaths, but that spike would result in a quick drop, ending the issue, as those who survived are becoming immune.
Also note, smallpox and polio are only two diseases. There was no vaccine for the Black Death (there is now, note the case in South Carolina), but it is still debated as to whether it just went away or herd immunity was a factor. The fact that the bubonic plague had similar statistics (inflate cases and deaths, they hit the marks) and just disappeared in one of the most unsanitary environments of all time is testament to what the immune system can do on its own. Sure, many diseases are ended by vaccines, but countless others aren't.