The fate of marriage does not depend purely on the happiness or otherwise of the partners within it, but the possible outcomes of ending it. In previous eras, to be divorced was shocking. People did not wish to be described as 'a divorced man' or ;a divorced woman'.
It sounded somehow louche. When divorce became more easily tolerated, and the church's role cannot be ignored here, it was not so unusual and so not so shocking. People stay within marriage for various reasons apart from love: financial reasons, effect on children, dependency, and therefore the number of people with 'lasting marriages' may belong to the list of happy marriages.
Who knows, but it seems likely that the proportion of marriages that are mutually happy are pretty much the same today as they have always been. It is just that, when unhappy, we can now get divorced easily.
I disagree somewhat with the statement that marriages don’t last anymore. My marriage has lasted 42 years but that’s probably because I am obviously from a different generation than today’s so-called millennials. But, even my generation, the so-called baby boomers, had fairly large divorce rates. The interesting thing is that some of the reasons marriages don’t last today are quite different from those of the baby boomer generation.
For example, social media didn’t exist for my generation but I believe it can be blamed for many failing marriages today. People just don’t connect physically. They seek attention from strangers, and as one blogger wrote, “Social media just invited a few thousand people into bed with you”.