But you don't need to extrapolate. Current rates are far more than sufficient for catastrophe. It's merely that the effects of dangerously high fertility rates are immediate while the effects of dangerously low fertility tend to lag the advent of said reduction by around 60 years.

So for instance Japan is still living the good times relative to what's yet to come since they collapsed in the 80s. South Korea hasn't even begun to really feel the consequences of their actions since they only collapsed in the 90s. And places like Finland or the Western world in general only collapsed even more recently.

But collapsed they have. And because our fertile window in life, for women at least, closes long before we die - most, if not all, of these places will, unavoidably, see dramatic population declines, screwed age ratios (with consequent impacts on the labor/retiree pool/costs), shrinking economies, and so on. And we're left to rely on some ever smaller generation(s) down the line to start having large families in this context.