Everyone is, yes, different. Being a secondary worked, in some ways, "better" for me when I still had a primary relationship (or any other romantic relationships). Right now, my secondary relationship with my partner is my ONLY relationship, and I don't know that it's incredibly healthy for me. It puts pressure on my partner, because he feels guilty if I'm having a rough physical or emotional time and he can't or chooses not to be here (in addition to being married, he travels a lot, is a very busy professional and devoted to physical training).
So yeah, there are times when I really wish he were here, and he isn't. Can't say those are always great days.
But two things that I think are really important:
1. I had plenty of days when I was married that I felt terribly lonely and like no one was "there" for me. That REALLY sucked.
2. Because sometimes I wish for more doesn't mean I want his marriage to blow up.
I love my partner deeply. I want him to be happy and fulfilled. How could I wish for anything that would cause him so much pain? Just as I believe his wife sees my value in his life, I see hers. They are plain, flat-out head-over-heels in love with each other even after being together 20 years. In many ways, she suits him as a primary companion FAR more than I would.
And even if I were to sweep all that aside--which I can't imagine being able to do--I think it's still true that if something horrible happened like his marriage going off the rails or, God forbid, something happening to his wife, there would be no guarantee and I believe no real likelihood that he would magically be my primary.
We work really well together part-time. I've never spent more than 6 days in a row with him--these days, it's usually a couple of days once or twice a month, less if he's on the road.
Because of that, we get to almost always be on a honeymoon. It's always special. There's that delicious rush of "not quite enough time." We have a real-life relationship that includes and encompasses our respective ills and ailments, problems, annoying habits, etc. But we don't live with any of that every day. We don't have to have money discussions, or household tasks, or stuff like that.
So I lose out on the day-to-day coziness of living with someone I love. But I gain space and time for personal growth and evolution, for rest, for parenting my son. And I get several days a month that are the most magical days of my life, even when we're just sitting around the house.
All that said--and I know this is long--I am guessing there are secondaries who struggle harder than I do with being a secondary, as well as secondaries who love it more, too. There are probably secondaries who wish they weren't secondaries, and maybe even misguided, confused, ill-intentioned secondaries who want to sabotage the primary relationship.
But I think what you really wanted to know was: is there an actual chance in hell that anybody would be content with being a secondary?
The answer is yes.