There is plenty of evidence of natural immunity decaying. It will likely never (not in a lifetime) fall to a level equal to an entirely unprepared immune system, but the same holds for vaccine effect: immune system responses are a log decay, with various nonlinear effects applied (thresholds, saturation, the occasional rare ADE and so on) to different aspects that are of different importance for different viruses so that the observed curves look a bit different in each case. Yes, for some viruses, even what's left after a century is enough.

But those details are completely besides the point for endemic state, endemic state is endemic state wether reinfection, on average happens every quarter or wether it happens every five years. The only difference is that anything larger than a few months will lead to massive seasonality, and longer cycles will lead to a larger severity spread. This severity spread will be the decisive factor in the question of revaccination thresholds (maybe higher than current flu shot habits, maybe lower, maybe about the same), but there are far too many uncertainties to bother with concrete predictions.

What we do know: a year or two from now there won't be any vaccine-only immunities left, everybody will either be vacc+infection or infection only.