COVID spread is an exponential process - you can't possibly do anything to stop R_t dropping below 1 from time to time. It saturates then recedes until a new variant appears. It it could possibly stay above one, we'd literally all have multiple cases of COVID right now. And I bet it does that fractally, so particular populations are all saturating and then seeing cases recede all the time.

The thing you're avoiding talking about seriously is the part that isn't already a certainty. Ie, the entire question.

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.