A decline in the marriage "rate" isn't the same thing as a decline in marriage at all. I don't think there is since the vast majority of people marry at least once in their lifetimes. It is rather that people have rearranged their lives to where they don't marry right out of high school or college anymore but perhaps have a series of relationships prior to marriage or between marriages. Sometimes these result in children, sometimes not.
A seeming increase in single parents doesn't change this fact. People have been bellyaching about this for decades.
That isn't the same thing at all as saying marriage is a thing of the past.
From somebody who has lived her entire adult life as a single person, here is the unvarnished truth: Because our society values marriage, people will still do it, and single people will still be at an economic disadvantage thanks to almost all married households having two incomes. That second income is a tremendous economic advantage over anybody who remains single, and it continues well after retirement. A living together situation is not the same thing as somebody who has lived his or her entire life alone and is self-supporting or is the same thing as somebody who has married. Most people who live together don't remain single all their lives but either eventually marry the partner or marry somebody else. Many if not most "live togethers" use it as a trial period before they eventually marry.
I really don't think many people truly understand how much the economy has changed to where it is absolutely devastating if you find yourself in your fifties or sixties, and you don't have the advantage of a working or a retired spouse. It is very difficult, very scary shit when you rely only on yourself and nobody will hire you because you are considered "too old." It is even worse for women because they are already at a disadvantage economically thanks to discrimination in the workplace.
The sixties' and seventies' move toward more choice in how we live our lives didn't forecast the fundamental changes in our economy to where it is virtually mandatory you have to be married or at least have a working partner in order to survive. Unless you are a trust fund baby, it is almost impossible to survive as a never married adult in this country.
I don't regret being single, but it is virtually impossible to survive without another income to cushion the blow.
The second income is really the only true advantage of being married to being single other than the tax advantage, and it is significant.