Right, but you surely have also seen many monogamous relationships also end badly.

Both of those examples could easily have happened with monogamy.

I could rattle off a bunch of poly relationships that have gone well. I know some folks that are raising a kid. They both have other partners. They're all pretty happy. Been so for years

Another couple has also been together for years. The one of them has had other relationships for years while the other focused on her career. For the last year or two she's got more time and is exploring dating. She's having a blast. They're all very happy.

There's a friend I've known for years that's done poly the whole time. He's had some breakups over the years, but that's just normal relationship stuff. None of them were to my knowledge catastrophic.

I can also rattle off monogamous relationships that went badly.

It's absurd to be like "monogamous relationships sometimes end badly. Poly relationships sometimes end badly. The poly ones are due to poly."