You miss the point-
The plague, small pox... none of these diseases killed everyone, and they all came and then left once a high enough percentage of the population had been exposed.
***Herd immunity is a reality and works even without a vaccine.***
The plague for example killed about 1/3 of the population in Europe in it’s worst wave, and small pox killed many native Indians, but never did these diseases kill everyone.
As to vaccines. They work in some cases but have gotten far more credit than they deserve (over sold in modern times by the push to get people vaccinated by government and the medical establishment). Many of the most lethal diseases out there have been eradicated or reduced because of modern sanitation, plumbing and refrigeration NOT vaccines.
https://www.cdc.gov/plague/maps/index.html
These diseases still exist. People are not vaccinated but they do not spread because of the conditions we have created. Which brings up an interesting inverse correlation that exists between the lethality of a disease and its ease of spreading (how contagious).
But you do what most people do, give vaccines the credit for what your plumber, refrigerator, pest control, food inspector, sewer, Army Corps of Engineers, trash man and dump have accomplished for you: a massive reduction in diseases.
In some cases vaccines were highly effective, such as with polio or small pox, the latter which you mention, but today these examples are used where they are not applicable, i.e. vaccines with a marginal effectiveness such as the seasonal flu. Not all vaccines are very effective. Likewise, arguments/ideas like herd immunity will be used when selling vaccines, but will seldom be mentioned now since it does not fit the narrative. Everyone will twist this corona virus into what they want it to be.
However, none of that changes the fact that once enough people are exposed, this virus will go away, all by its own. Even without a vaccine once 50 - 90% of the population has been exposed, the disease disappears because the paths of transmission no closed. Without a vaccine in hand, when we created a mass (blanket for all) social distancing policy and shut everything down, ***we only delayed the inevitable.*** The logical solution would have been to isolate a small at high risk population and let the disease run its course (the opposite of what we did). Once the disease passes then those at risk are safe and can come out. Sweden had it right-
Smallpox didn't just go away. It came year after year for 2,300 years. Sometimes it killed millions at a time, sometimes tens of millions. In its last 100 years, smallpox killed an estimated 500 million human beings. It didn't kill everyone on Earth, but it wiped out huge swaths of people. Polio didn't kill everyone on Earth, but year after year outbreaks left many people dead and many more crippled for life; most of them children. It absolutely terrorized parents for generation after generation. You think a parent who's lost a child to polio cared that other kids lived? You think someone who is crippled for life is thanking their lucky stars?
If the plumber and the food inspector and the trash man were all that were needed to eradicate disease, we wouldn't have any diseases anymore. Yet here we are, still dealing with diseases, including new ones like SARS-CoV-2 that's killing over 130,000 Americans. What we need is a constant herd immunity to help contain outbreaks. The quickest and least painful way to achieve herd immunity is with safe and effective vaccines that are widely deployed.
Each of these diseases was brought to its knees at very different times. All of those times just happen to be directly after widespread vaccine use. You really think every single disease we vaccinate against just decided to surrender at that exact moment for no other reason? Smallpox decided after 2,300 years to just disappear from the entire Earth for no reason, unrelated to the massive worldwide effort to vaccinate everyone against it?
What an amazing coincidence! That after 2,300 years killing hundreds of millions of human beings, smallpox vanished from the face of the Earth after a 9 year smallpox eradication effort using mass vaccination!