We don't know what the workplace will look like in 28 years, or what that will mean for economic productivity.
There's a good argument for having a birth rate marginally lower than the replacement rate for a few decades (a manged decline in total population) but we certainly don't want a cliff-edge drop we have no idea how to face.
And even if it were the case that, in the future, 1 child could support multiple retirees, a society where the majority are elderly and there are very few children does not sound an ideal one, to me.
Based on current birth rates, more than 50% of South Korea's population will be aged 60 or older by 2100, and only about 11% will be children, and the shift will continue to accelerate.
Yes, that's a potential economic disaster, but also a disaster for national defence, culture, and the basic social fabric.
Things arent quite so severe in the UK, yet, but it is well along that path (as is most of the West).