I am someone who experienced the "longterm partner decided she wanted to be poly" heartbreak. When she told me, I asked her why she would choose to stay with me rather than just be single and date as many men as she wanted. She told me something along the lines of "I love you, I want to spend the rest of my life with you, but I don't feel like one person is enough for all the love I have". I wasn't terribly happy with that response, and she broke up with me a week later (while a man who she wanted to be poly with was staying with her). She left me because she wanted to change the parameters of the relationship and I didn't
Afterwards, oddly enough, I wound up in a friendship for a while with a different woman who had just broken up with her fiance for almost the same reason. In her case she had always been open about wanting to be Poly, her fiance had been okay with it, and I guess changed his mind the closer they got towards marriage. She left him because he wanted to change the parameters of the relationship and she didn't.
I wound up talking to the second woman a lot about polyamory and my unhappiness with how my ex had treated me. One night we met for coffee and she basically spent the whole time trashing me. She called me a loser for still being upset about my ex, she told me I was a miserable sad sack of a person and I needed to get over it, etc. Once she was done with that, she proceeded to tell me (in unwelcome and unwanted detail) about a lot of the latest sex parties she had been attending and how excited it made her to be living her fantasies. Then she casually asked if I wanted to go back to her place and screw (which was not our relationship up to that point). I declined, told her I didn't want anything to do with her anymore and left. She spent a couple of weeks asking me what she had done wrong. I mostly ignored her but even when I tried to explain she just argued with me, then eventually she cut me off with a long tirade where she acted like it was her choice not to have anything to do with me anymore, not mine
I'm aware that n=2 is not statistically significant, but those two encounters happening within the span of a few months kind of convinced me that people who are Poly are self-centered, emotionally stunted, and absolutely not suitable for any kind of long term relationship
Yeah a lot of monogamous relationships end these days too, but if this is representative of even a small fraction of poly people, I wonder if you can even call poly a relationship at all, really