R_t fell under 1 when the population was still naive. If you don't let the virus proliferate massively to begin with you won't have variants that are adapted to human immunity.

It's a certainty because it happened. R_t fell under one as a result of NPIs, not immunity.

You don't have to keep R_t under 1 forever. Just for long enough that the virus fizzles out. It's possible and in fact it has been done repeatedly.