It is very simple.
Every single established epidemic in the past always was ended by herd immunity.
Whether it was Polio or Ebola, the infection got too greedy and used up all of its local hosts, so then died out.
That is also what ends every seasonal flu.
But with covid-19, we prevented that by "flattening the curve".
Which then prevented the spike that normally burns out the available local hosts.
And then by conserving hosts, that allows the epidemic to last essentially forever.
So instead of 50k death in March, we flattened it to 30k per month, forever.
Not a smart move.
We got a vaccine for polio, I don't remember there ever being herd immunity for it or Ebola. What?
No the last Polio epidemic was in 1948, and we did not get the Salk vaccine until 1957, after it was 95% over.
With all epidemics, not just Polio and Ebola, it was herd immunity that ended it, like Hong Kong flu, Asian flu, Avian flu, SARS, MERS, etc.
But a vaccine also relies on herd immunity.
Any time you do not need 100% of the population to be immune and still kill off the pathogen, that is herd immunity.
Ebola also is fought with quarantine, but what ends it in the quarantines area is herd immunity.
The virus is too greedy and is so infectious that is uses up all the available local hosts too quickly.
They it runs out, and dies.
That would have also happened to covid-19 in March, is we had not deliberately conserved hosts, so that the virus would not run out, and could last forever.