When people talk about poly relationships, one of the more common approaches seems to be the one where the goal is for everyone to get "their own" relationship with a potential partner. For me, while each partner and relationship is unique, they become a part of the overall construction of my life, and I theirs, not a stand-alone structure.
This is actually one of the things that I appreciate about poly relationships: context. Sure, everyone has a life, and circumstances that make up who, and where, they are in their journey, even within monogamous relationships. Within the poly realm, there is often a very rich environment that I get to become a part of, rather than creating everything from the ground up.
Try visualizing it this way: If each person has a structure that represents their life, when someone decides to become a partner to another, their structures get closer together, maybe even merge. A new partner means my house might get a new wing, or an upper floor, a cool, expansive basement, or maybe we opt to build something new from the ground up. My metamours can be reached by walking through our shared partner's space, and visiting. Each person that decides to join the larger relationship brings who they already are to the party, and what started out as a simple cottage may become a sprawling mansion over time.
There are certainly poly people out there that would prefer to stay in the guest cottage, or just want to park their RV in the driveway for a bit, then leave, but for me the real win is when I have the opportunity to build something bigger than what I can manage on my own, and get to join my structure with someone else I love, and they want to do the same. It doesn't mean we don't get our own relationship, it means I become part of something more than just "us". Maybe, if I'm particularly fortunate, I get to expand my family.