So long as the children of immigrants are adapting fine to the rest of American culture, who cares if our population growth depends on them?
It was difficult to read this and not laugh because it conveniently leaves out First Nations, as well as the fact that the USA in particular used immense amounts of slave labor while the "natives" you refer to (the whites) reaped the rewards in relative comfort.
And many (most?) Western countries do provide various incentives to have kids, as well as Asian countries, but the incentives don't ever seem to work to get above that 2.1 replacement level, so Western governments are 'worried'/concerned about birth rates, but boosting them significantly not an easy thing to do. Especially when part of the reason might be that people simply don't want more than 1 or 2 kids these days regardless of finances.
Its not just about the number of children per women/couple: the missing element is to have children really young.
Like 16 - 24
Make childcare, schooling, etc tax deductible for starters.
Not just because of costs, but the harder to modify cultural reasons like people valuing independence, "casual dating"/FWB more common instead of dating with intent to marry, less social pressure/shaming from parents ("why don't you have a husband/wife/kids yet?!"), freedom to "find yourself/travel" or "get sexually experienced/have a ho phase before settling" for years after college, etc.
Most people don't want to go from college to marriage and kids right away these days, and a few years of this type of casual dating/exploration after college eats up a lot of prime reproductive years like you said. But these cultural factors are very hard to change especially with the absence of religion/strong communities these days.
At this point it affects most countries outside of Africa.
immigration
I believe in diversity. Immigration will forever change the countries it affects, typically making them more like the source countries.
boosting hard
One might be able to push it just above 2.1, but fundamentally the causes are significant, unprecedented technological and social changes since the industrial revolution. Nowhere is it guaranteed that these changes produce a stable, lasting order.