Marriage rates are also there, so that statistic is not really that surprising.

Most of the people who would have been getting divorced in the 1970s, today look like a relationship that fails after a few months or years, before reaching the point where they would have decided to get married.

That's not a complete argument though (not to say the conclusion's wrong, I have no idea) - what's happened to marriage, non-divorce separation, death, and birth rates?

US marriage rates are at a 120 year low.