Even if models predict the population growth will slow down and may turn to a decline after peaking 50 years from now, it feels quite rash to over-extrapolate and panic now about a potential eventual decline from the 15 billion peak to a genetic population bottleneck (1000-ish). The population would have to halve ~25 times which would take ~1000 years if it declined at a similar speed as it's been growing now.