A very good reason to wait until your 30s to have kids: Combined incomes are finally enough to meet basic living expenses.

    This is a shift away from informal family-based
    responsibility for raising children, where parents
    followed their intuition, ...

Informal family-based responsibility was a byproduct of extended families - back when extended families were common.

    to a more child-centred, expert-informed approach, 

My wife and I were fully on our own for our first child. With no extended family to draw on, we got knowledge wherever we could.

    where parents pour in more time, emotion and financial
    investment to ensure the success of their children
    for which they feel personally responsible.

Because parenting went from a few hours a week with extended family support to 24/7 adulting w/ no support.

When there is no life beyond your children, it turns out that rat-racing them toward a bleak future is the best of the bad options.

It will be interesting to see this play out over the next few generations.

The theory is yesterday declining population levels mean that the tax base shrinks just as the larger, older, generation retires. So "paying" for more kids is kinda like a long-term retirement investment.

But Norway has a sovereign wealth fund. So gets a chunk of revenue from not-taxes. Does that alter the equation?

Also housing is expensive. But as population numbers slowly fall it (presumably) gets cheaper.

Immigration is another strategy to bolster populations, but Noways's climate, not to mention cost of living, does not make it especially appealing to most immigrants.

The real question remains, to what extent will older people actually suffer, as populations decine to a stable point? There are lots of dire predictions, but not much experience.

Why not just introduce a competitive, market-based system and let the market decide? Give a monthly subsidy to every woman for having a child until the age of 18, regardless of marital status and number of children, and fund it by a flat per-head tax (flat as in, in $, not %). Adjust amount of subsidy as the birthrate changes to keep it at the optimum level, locking it in (inflation adjusted) till kid reaches 18. Poor women will be compelled to have kids because they can't afford the tax. Because they can have many of them, easily 5-6, and it will immediately become competitive as subsidies fall as the market is saturated (we all know too well how any market with no entry barriers and guaranteed pay works - it is immediately inundated by supply, see Upwork) - the resulting fee will be very low. People who refuse paying tax claiming no income - send them to work (women) or military service (men). In a poorer country that could result in mass emigration, but Norway is rich and no one in a sane mind will escape it.

Problem solved.