I left polyamory after 25 years for none of these reasons. I just finally had had enough of spreading myself too thin, being repeatedly backburnered or “transitioned” during a partner’s NRE, and settling for mediocre compatibility or other dubious traits because I only see a person once a week.
In a highly compatible life partnership, I’m polysaturated at one person. I may be capable of loving more than one person, but I don’t really enjoy juggling the needs of more than one romantic relationship, and I have the scars to show for it. My life is too busy and full as it is, and I barely have time for friends and hobbies, much less another partner. I also find that having multiple partners forces me to try to grow and build my life with them in two or more, often opposing, directions at once. I prefer to give and receive the depth of time, focus, priority, and attention only possible in a relationship that is consciously monogamish.