I think where you're going wrong is you're expecting people to cheat because they're trying to "trade up". You're thinking they want someone better. Smarter, prettier, funnier, younger.
In my experience, people cheat not because they want something better, but because they want something new, different, or just want more.
My Dad cheated over and over again on every woman he was with because he felt a desperate need for everyone to like him. He'd neglect friendships because he was chasing after a potential new friend, and he did the same with every relationship because he needed the next person to love him. It didn't actually matter what that person was like, he just needed them to be into him. He always wanted what he didn't have.
I on the other hand used to cheat because sex with someone new was exciting, and I didn't really care about blowing up what I had in the process. I was young, stupid, selfish, and the role model I had was my Dad, so no help there. I enjoyed the relationship I was in, but if there was a chance to experience someone new, then I'd always go for that, because what if that chance never cropped up again.
And then I met DP, and properly fell in love for the first time, and in 20 years haven't cheated on her once.