You made the point that the only reason that developed countries don't have as many children nowadays is because of the increasing costs vs income. That's what "If they're looking for reasons the birth-rate has plummeted in developed countries they needn't look further than money" means.
Yet all the analyses from across the globe point to higher education, gender equality and freedom of choice as the reasons people don't have as many kids... rather than money. And the developed world is not an alien species on a different solar system. Rich vs poor analysis is not out of context here, just because you didn't specifically make a judgement on it. It's yet another very strong indicator that money alone isn't one of the prime drivers of this fertility drop.
Undoubtedly, those things you mention are factors for decreasing birth rates - they just aren't as relevant to developed countries
today as they are to developing countries. Global trends that are leading to decreases in birth rates (e.g - changing attitudes, the decreased economic utility of having large families, the availability of contraception etc.) occurred decades or more ago in most developed countries. In the UK there have been two major historical falls in birthrate which correspond almost exactly to the current trends you are identifying in the developing world. The first one was rapid industrialisation and the advent of child labour laws in the late 19th and early 20th centuries. The second one was a cultural shift combined with increased availability of contraception in the second half of the 20th Century which lead to a further collapse in the birth-rate between the mid-60s and the early-80s.
Since the early-80s the birthrate has been relatively stable here until the last few years where there's been a downward trend again. My argument is simply that the reason birth-rates are decreasing
in wealthy countries at this particular moment in time is because 20-somethings now either flat out can't afford them, and because general economic trends mean that they are less likely to lead lives conducive to 'settling down and having kids'. Obviously changes in attitudes are still ongoing but you'd be hard-pressed to demonstrate a statistically significant shift in the last 20-30 years or so to explain the fact why the current raft of 20-somethings aren't having kids. The one thing that is demonstrably different between the 20-somethings of today and the 20-somethings of the 80s and 90s in that 20-somethings today are, across the board, less likely to have stable lives.
That's a really simplistic view, though. Cost of living has increased but so has the standard of living, housing, medicine, education etc. for children. Your view doesn't make a lot of sense given that historically, the poorer people are, the more children they've had. This is based on a myriad of factors beyond costs, such as lack of access to contraception, religious beliefs, and people just wanting more kids to help with low skilled jobs like farming.
I would argue the main reason people in richer countries have less kids these days is opportunity. People can afford to travel the world now, or get degrees, or get high skilled professions, or just live a far more enjoyable and comfortable lifestyle that they likely don't want to give up in order to have children. People also get married and settle down at a far older age, so the "window" for actually being able to have kids is much smaller.
I've addressed some of your points (especially your first paragraph) in my response to Mike above so sorry if there's repetition.
Life is definitely more comfortable for the majority now (not sure about enjoyable, work-life balance has taken a hammering in the last 30 years and we have a tendency of looking back at the absence of modern conveniences in terms of how we'd feel if they vanished, rather than how life was before them), but this has little to do with the amount of disposable income that the average 20-something has. A key point on the bolded is that getting a degree or being highly skilled is no longer a guarantee of financial security, never mind wealth. Someone with my job in 1980 would be able to sustain a middle class lifestyle: nice house, nice car, holidays etc. for themselves, a stay-at-home partner and a couple of kids. In 2020, my fiancé and I would probably need to earn twice as much as we do to get close to that standard of living.
The issue is far less that the lives of 20-somethings are too good to abandon for a child, it's that they aren't lives many would contemplate bringing a child into. Not because they're necessarily bad (although as I've said, the cost of living means that it's perfectly possible to be working full-time and still struggling), but because they're
insecure. Millennials are far less likely to have a permanent contract, far more likely to work part-time hours/shift work, far more likely to have to relocate away from their support systems for work and far less likely to own a home than their parents were at the same age. Many people will move house half-a-dozen times in their 20s, hold several different jobs and live in a few different cities. Again, these aren't necessarily bad things, but living a sort of semi-nomadic lifestyle where you don't know where you'll be living or whether you'll be working when your contract ends is not conducive to settling down and having kids.
Undoubtedly, some people enjoy that lifestyle and lean into it, but the reason people are settling down later is because most don't reach a stage in their career where they can afford to do so until their 30s. Although, I think that's especially acute in my peer group because the job market didn't recover from the crash until most of us were in our mid-20s. Associated with that is the fact that most of us didn't have the taste of freedom and disposable income our parents got at 18/19 until we were in our mid-20s, so it that sense it's a bit of a chicken and egg between the financial circumstances and the different priorities.