>> This new equilibrium is different from the one in the past when it was the very high mortality that kept population growth in check. In the new balance it will be low fertility keeps population changes small.
Culture. It's principally cultural not biological. It's perhaps clearer to say it is a decrease in fecundity rather than fertility.
Worldwide experience has been that increasingly developed nations have lower average fecundity. There are various likely reasons but the correlation seems to be quite robust. In most countries it seems that the causal link was from development to fertility. It is arguable that China reversed the causation, politically limiting fecundity.
This is a uniquely human dynamic. Pretty much every other species in the world drives its population to its Malthusian limit.
Excessive taxes and expensive housing. In much of Europe it is simply impossible to buy a house on a single income, if you obey 100% of tax laws. These taxes are used to pay for the elderly, their welfare (pensions) and healthcare.
If you are in the US you may not appreciate how heavy the tax burden is in Europe. It varies by country but its normally something like: 30% payroll tax paid by employer. 30% income tax paid by you. 20% sales tax paid by you. Then, very expensive housing due to the high population densities and limited room to expand.
Additionally in Europe, many parents want their children to be educated in English, however the language of instruction is usually the local language. This is partly why the migrant stream into the Anglosphere is so high. The alternative are private international schools, which are hideously expensive.
What is the reason for low fertility prediction?