I dunno, mostly that this has all been done before and it isn't the end of anybody's world. That long-term, loving friendships can grow and change in unexpected ways and still be maintained without damage having to take place elsewhere. Sounds rational and possible, but this is all incredibly new to me.
Well, it is true. There is nothing new under the sun. Honest. What are you afraid of losing to "damage?"
Keep in mind that stable relationships, whether monoship or polyships or something else are usually just being their regular ol' normal selves doing the business of living -- groceries, making dinner, going to work, raising kids, walking the dog, going to church, whatever it is they do. They aren't shouting from the corner "8 o'clock! And all's well!"