It's bad if it goes down by more than about 1.2% per year. That would mean zero births, present-day natural deaths. Of course zero births isn't presently realistic, and we should expect the next 10-30 years to significantly increase human lifespan. If we assume continued births at the lowest rates seen anywhere on the planet, and humans just maxing out the present human lifespan limit, then anything more than about a 0.5% decrease means someone is getting murked.