There's two ways to predict the future. The first is to really understand the present and the second is to make the future happen. Let's start with the first.
Why is marriage declining? To understand it, you need to know two facts. First, CHILDLESS marriages are breaking up. Marriages with children aren't breaking up so much, at least until the children are grown.
Second, poverty is going down and wealth is going up. Which means people have more opportunities and more freedom. And with this comes mental freedom. One of the four indicators of mental development has gone up.
It used to be that all people cared about, literally all they cared about, and all they could AFFORD to care about, was their family and their workplace. And NOTHING ELSE.
Nowadays, people can afford a great deal more principle. Even maids and servants are perfectly comfortable changing employers. They're no longer operating on the Subcomponent level, but on the Submissive level.
Where before, a maid or servant was a part of their master's household FOR LIFE, nowadays they're not part of the household period, and are just hired for the duration. They'll be the maid or servant of someone completely different in 3 or 7 years' time.
When you put those two facts together, the fact that people are having fewer children, that children STILL matter, and that people are growing mentally ... you come to the conclusion that marriages are breaking up because people have more principles now and simply *can't stand* each other.
Why should a Communist tolerate a Nazi? They would have in the past. Nowadays, it's not that the proportion of communists and nazis has gone down at all. If anything, it's gone up. Rather, they can nowadays afford to hate each other's guts, and they've grown enough to WANT to hate each other's guts.
So how will society develop? How will marriage develop in the future? It's pretty fucking obvious. People will adjust by pre-filtering their spouses looking for people they can stand rather than relying strictly on looks and sexual attraction. And doing this will entirely reverse the decline of marriage.
The only point that requires my expert judgement to address is the time horizon this will occur on. And in my expert judgement, I give it 50 years.
Finally, how do I know that marriage won't dissolve or mutate into something unrecognizable? Easy. Because human brains will always, ALWAYS operate on the PairBond level in conjunction with any other levels they operate on. Because EVEN AI will operate on the PairBond level. Because the PairBond level is one of the 11 meta-levels that can be formally and rigorously defined using OOSA. And while this is more than enough to anyone who's seen the math, there is the separate fact that mathematics doesn't allow stable solutions to the man-woman-dog matching problem. In other words, because math says so!