I don't want to disagree with you too vigorously by getting caught in the weeds on the specifics of marriage (I disagree with you there, but it's not worth covering at length here). I think the diminished marriage rate and unhappiness of the unmarried are both symptoms of societal problems and useful heuristics for poor outcomes. The decreased marriage rate is consistent with the impression of a society that's more disconnected, lonelier, and sadder than previous generations. This ties in with diminished sex rates, increased rates of depression, and other negative outcomes.
I don't think marriage is a silver bullet for millennials. My earlier sardonic reply to another poster that kicked off this line of discussion was:
It's not just marriage though - substitute "go back to forming normal, healthy pair-bonds" or "normal, healthy lifelong friendships" and the point is made even more strongly. All of these are problems.