What posts are these that are saying that nobody is irreplaceable? Personally, I would completely disagree.
I'll use my girlfriend as an example because her situation is closer to yours then my own. She and her husband have been together for 14 years. If she lost him she would be beyond devastated... they play vital roles in each other's lives. She also loves me -- we've been together for over two years now and I think if we broke up it would take her a long time to get over it. Her love for me does not diminish her love for her husband or make him in any way replaceable. To her he is irreplaceable and always, I think, will be. I don't know if she and I will ever play as deep a role in each other's lives as she and her husband do but we're open to the possibility of some day of being that important to each other and it would in no way mean that he would be less important to her.
Relationships are absolutely not interchangeable whether you're mono or poly.
An excellent article on the subject by Franklin Veaux, "Polyamory, Loss, and the Superhuman Soul": http://www.xeromag.com/fvessay06.html
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.