OP had a hunch and I provided something that wasn't just an opinion. If you then looked at the literature you could have pointed out that some research support an erosion theory of relationship satisfaction. I could have answered back that there is evidence that relationship satisfaction is a huge factor for relationship stability (who would have known), and stability should support relationship longevity. In the end we wouldn't be that much closer to a sure answer to the question, but we would all have learned something worthwhile about the topic.

It's a large and sprawling field of research. Why don't you dig into it yourself?