Regarding treatments, please keep in mind that there are no effective treatments for viral diseases.
The available treatments can save people from dying and possibly reduce the seriousness, time in hospital, etc, but none cure the disease as such.
In fact, the path to drug development is probably longer and more difficult than for vaccines. The most notable exception will be HIV, because the virus has a very high mutation rate and it is incredibly difficult to design consensus targets for antibodies. And that HIV treatment involves a drug cocktail that took years to develop.
Vaccines are much easier, as it turned out. And as everyone expected, which is why there was a huge drive to produce vaccines, which are now being used. Whereas treatments, much less so, despite media publicised promises.
So, without vaccines, yes, we would have to rely on almost everyone getting infected at some point. Maybe.
We also have to keep in mind that we still do not know how long immunity from having had covid will last. And that in high transmission settings, with a large number of people infected, new variants can (and did) emerge, which can then evade the immune response to previous variants. So, the so called herd immunity might not have worked.
It is very hard to predict how the virus would settle in the population without a vaccine. That is the subject of evolutionary virology and molecular epidemiology, and not sure most virologists are capable of making any reliable predictions.
I'm more with Dr. Ian Malcolm on this one, chaotic systems are rather difficult to predict.