1. Why do you believe what you believe? What would change your mind?
I think it’s pretty normal to want to have sex and romance with more than one person, and pretty normal to feel jealousy over your partner having sex and romance with other people. These things are obviously in tension. I think how you resolve this tension is a very individual decision, so I don’t think you can say either polyamory or monogamy is better or worse for people in general. And I’m not sure what “society” has to do with it.
I think the “what would change your mind?” part of this question is very strange. It’s like being asked “what would convince you that homosexuality is worse for its participants and society than heterosexuality?” To get the question to even make sense, you have to imagine living in a very different world than the one we actually live in.
2. A monogamous person has a crush on someone other than their partner. In a healthy relationship, what would happen next?
I don’t want to say “nothing” like that’s the only healthy response, but… not necessarily anything? It would definitely be a big mistake to treat it as a huge relationship crisis. Expecting your partner to never have crushes on anyone else is not realistic. The big question, I guess, is whether you should tell your partner. While I try to practice something close to radical honesty in my primary relationship, I realize this isn’t for everyone, so I think keeping your crushes entirely to yourself is fine, maybe even the thing you should do by default if your partner hasn’t explicitly said they want to know about your crushes. On the other hand, I don’t think it’s inherent in the nature of monogamy that you can’t have fun talking about who you might want to date if you weren’t together.
(This is all assuming you have good reasons to be monogamous. If you just defaulted to monogamy because that’s what everyone else is doing, that’s another matter entirely.)
3. What would happen if 90% of people in a society were polyamorous? (You may assume they all practice one style of polyamory, or different styles.)
It would probably be pretty annoying for monogamous people. They’d have to go through life knowing that the random cute person they happen to meet one day probably isn’t monogamous. Probably a lot would respond by leaning heavily on online dating, where they can mark on their profile that they’re monogamous and filter for people who do the same. Others would start monogamy meetups. Certain hobbies might become known as hotbeds of monogamy, good places for single monogamous to meet other single monogamous people. Silicon Valley might see a proliferation of nice corporate Slack channels and Google Groups dedicated to monogamy (perhaps James Damore would be happier in this universe).
But on the whole, I expect the impact would be smaller than many people expect.