If the definition of "success" for a relationship is that the relationship lasts until one of the two people die*, then the majority of monogamous relationships fail.

So lets use the U.S. as an example. The average age for a first marriage is late 20s. Most people have already had a few monogamous relationships that "failed" prior to that marriage. Often starting in high-school or college. A marriage is rarely anyone's first relationship. And still, about 30% of marriages end in divorce. Usually after 7-10 years. 10% end in the first year.

That means most people in their 40s have multiple failed monogamous relationships and maybe one failed marriage. Even the ones who get married and stay married until death have some failed monogamous relationships and one successful one. So more failures than success.

And those people who divorce often go on to have more failed monogamous relationships. They rarely remain celebate until death. They date again. Maybe marry again. If they marry again, the divorce rate is even higher. So for every monogamous relationship that lasts forever, there are many more (most) that failed leading to that "success". And of course that definition of success doesn't account for happiness. Only longevity.

If most monogamous relationships lasted until death, most adults would still be with their first monogamous partner from high school or college and stay with them until death.

And yet monogamous people will cite the "failure" rate of non-mono relationships as proof that it doesn't work. While pretending or flat out denying most mono relationships fail. Like almost all of them.

*Not my definition of success for the record