) that *if* a relationship is to end, it's because the relationship no longer serves the two people involved, and the two people involved do not wish to continue, and NOT because of an external person or other relationship.
I don't see how this contradicts the point I made in the other thread.
Sometimes, a relationship no longer "serves" one of the two people involved because of the conflicting needs of another partner.
That's when a relationship that used to "serve" is in danger of no longer "serving". And this can be any one involved who feels this way.
A non-nesting relationship can work fine until the needs of any of the people involved change. It might be that a married couple have a baby, and now this additional lack of availability no longer "serves" their other partners. It might be that a non-nesting partner needs contact and time that just doesn't mesh with the needs of your spouse (a KTP vs parallel type issue) because anyone's needs can evolve over time.
A good poly saying is that relationships do not occur in a vacuum. It could well be the needs, choices and limitations of a third person that sparks the acknowledgement that a relationship is no longer serving the people involved.
You could "need" to move for a dream job that creates distance more than you need to give up the opportunity and stay within a practical distance for your partner and your metamour.
Your partner could find themselves "needing" to be nearer you than his other partner, now confronted with the choice to have less contact with you.
His other partner could find herself "needing" to be out of a long distance relationship because it doesn't "serve' them. She already struggles now he leaves for weeks at a time.
Somewhere in that is an ultimatum where your partner decides to either be near you, or appease his partner's need not to move away and effectively end their relationship given her need for local partners.
Doesn't mean anyone said "it's me or her" or that anyone wanted to effectively freeze the other out. It could be a devastating situation between three people who.care deeply about the welfare of the others. But it's the reality of what happens.
The only Real way to protect against it is to find people who seem to want the same type of relationship as you do and have only involved themselves with similar relationships.