And my point is that an “everything else being equal” argument about the income and spending of an individual is not particularly useful when discussing demographic issues.

Yes, the cost of things is more expensive in dense cities, but by 2x-3x for some services and not at all for buying a pair of jeans from a retailer - those prices are pretty much uniform everywhere, modulo state/local taxes. I'll argue that one's quality of life is better in SV on $250k vs $70k in the rural midwest or $100k in a midwestern city - obviously this will depend on spending patterns, which depends on personal preferences. But for me, it's a conscious decision to make more and spend more on a west coast city.

The problem is you need more space after having kids. If your already spending 75% of your income on rent adding another room is not feasible.

Or more basically you need a significant surplus to be able to afford kids.

That entirely depends on the income level you're talking, which is the OP's point. If you're making $10 mil and spending $7.5 mil on housing, you probably already have enough rooms to have kids. And even if you didn't, you could surely find room in your remaining $2.5 mil to add a room onto your house or find a slightly bigger place. This obviously does not apply if you're making $50k... Hence why absolutes are relevant.

You are assuming that 7.5 million buys a multi room home. At the extreme it may only rent you a 350 square foot apartment.

Some people might spend 75% of their income on massive dwellings, but most people downsize if possible before going past 55% of income. And as we are talking about things on the social not individual level those preferences matter.