As many people have said, it's happening in slow motion at the moment.
We also have real-life insight into what happens when numbers of fertile females decline. China's one child policy (now relaxed) plus sex-selective abortion have led to a defecit of women in the population. The response? Trafficking unwilling women across the border from neighbouring countries, industrial rape and forced marriages.
It would be fascinating I think to see a breakdown of what women actually wanted for themselves, in the absence of social or economic pressures/coercion. My hunch is that you'd get some sort of distribution within the female population where the majority wanted 2 children, with minorities either side who wanted either none (or occasionally just one), and minorities who wanted more than 2 (with most of those wanting just 3, but a few wanting very large families). I suspect that overall, left to their own devices (and able to afford families of whatever size they chose, plus continuing with jobs where they wanted to/stable relationships to provide a financially secure environment where they wanted to be SAHM), overall the female population would settle of its own accord somewhere round about replacement level (2.1 children per individual on average).
But I don't think any country in the world provides this absence of coercion/economic pressure. Italy, for example, has low birth rates because housing is so expensive couples can't afford to set up house together and end up staying in the parental home into their thirties. End result - couples finally get an appartment of their own, then have 1 child because that's all they can afford. I'm not sure, talking to Italian friends, that this is what they want, more what they feel constrained to do by economics. Germany has huge social pressures making it (relatively speaking, for a western European country) unacceptable to be a WOHM of pre-school children (women who do this are referred to as "raven mothers"), so I guess there are maybe more young German women who feel there's a choice to be made between career and motherhood.