Everyone is bringing up such good points!
I think that part of the confusion comes from mononormative culture where, yes, the success of a marriage is measured by it's length and stability.
Part of the poly lifestyle is to be flexible and adapt to change. Sometimes you have to let a partner go because their life takes a different turn.
How would I measure success in polyamory?
I haven't thought of it but I recall some examples from The Ethical Slut about how relationships come and go and how there doesn't really have to be any pain or loss. Maybe you haven't seen a partner for ten years for whatever reason, but you can still pick up where you left off. Or you can break up with someone and maybe 5 to 15 years later try again.
I think that it's inherently misleading to measure the success of a single poly relationship. I think that shows a fundamental misunderstanding of the lifestyle. I'm only seeing one person right now, but I'm living with her and her husband! Does that mean I'm not a good polyamorist? Since I'm only seeing one person? But everyone is happy and healthy, so I'd call that a success.