No, the collapse didn‘t ‘begin under Ronald Thatcher’, at least not in North America. In both the U.S. and Canada, the fertility rate plummeted beginning in about 1965; the total number of children born in 1975 was not much over half the number born ten years earlier, despite a substantially larger population of childbearing age. In the 1980s, the birthrate actually recovered somewhat, because the Baby Boomers, though having far fewer children on average than their parents, had some; many of them just deferred childbearing until later in life, so the ‘Echo Generation’ didn’t arrive in large numbers until a decade after they were first expected.

The rest of your argument I find quite sound and convincing. I just wanted to caution you about a subsidiary point where you have made a slip with one of your supporting facts.