Even though polyamory typically involves more people, it also typically requires a bit less dependence on other people for reassurance on things like whether they're invested. They either are or aren't - and you choose whether to stay based on whether you are or aren't, in my experience.

The title of this piece reminded me of a reddit post I saw recently where a woman in a long-distance poly relationship went without seeing her male partner for two years, then his new partner vetoed the meetup they finally managed to plan. She was devastated about the veto, but her post was littered with evidence that her partner had long ago lost interest in the relationship. (He had opportunities to see her during those two years, for example.) Losing interest in poly sometimes looks like what I call 'slow ghosting'...letting someone think you're still in a relationship while quietly investing more in your other ones.