Yes, it is. It's very difficult to shake the mindset of "He's replacing me" in favor of thinking "He's keeping me even if he adds others." But that's pretty much how it works for polyamorous people. They don't try to replace people, because it's impossible to replace anyone. They just enjoy connections with more than one person.
When I started polyamory-ing, my boyfriend Guy was primarily a long-distance partner. Because of that, and because they were sort of friends, Hubby didn't have a problem with Guy. When I started seeing S2, though, I was involved with someone who lives near enough for me to see him at least once or twice a week, and that was difficult for Hubby to wrap his head around at first.
What helped him was when I said "You aren't more important than him. He isn't more important than you. You're both important to me in different ways." As a bit more time went on, Hubby recognized that with S2, I was able to have needs/wants met that Hubby was unable or unwilling to meet, such as going to parties, taking long drives, and cuddling on the couch watching TV. All things S2 greatly enjoyed doing, and that I enjoy, that Hubby flat out refuses to even consider doing. Likewise now, with Woody...Woody has a wide social network. I want to have more friends. Hubby doesn't know anyone except me and the guys he works with, and prefers it that way. Woody helps me to meet my need to widen my own social network, because that's something Hubby is unable to do.
But Hubby is able to talk me down from PTSD meltdowns and flashbacks. He's able to read my body language to know how I'm responding to something sexual, sometimes even before *I* know. He has taken the time to learn what PTSD, anxiety, and depression look like for me, and to learn how to help me when those things impact me. Most importantly, Hubby is the one with whom my life is fully entangled; he works, sometimes two jobs, to take care of my kids and me because I am unable to work. Those are things he gives me that no other partner would be able--or probably willing--to do.
No one person can be EVERYTHING to someone else. But everyone brings something unique and different to each person in their life. There might be things you aren't able to give your partner that another partner can give him. But on the other hand, there are unquestionably things you bring to him that no other partner could, and that's why he wouldn't replace you.