I have good close friends, they are a little younger than me, in their mid 40s - in a poly relationship, perhaps the difference I see in their relationship is it opened up after decades of commitment, raising children and the trust and communication skills that developed with that task. In them, I see none of the illness, narcissism or duplicity you describe, but also they are not on apps, only their closest friends would know. The live their life and don’t go yelling poly all the time. I expect their quiet unadvertised experience is fairly common