I'll share my experience.
My ex and his AP both cheated to be together.
She is much younger than him and had she gone to uni, would just have finished this year. He is approaching 34. Because she never went to uni, she still lives at home and he owns a property.
I think, but don't know for sure that she probably had an unstable upbringing. Her dad has been married several times, she has a couple of half siblings, one of which committed suicide. She probably just wants a stable man in her life and if she does what my ex says, then she will probably get that. She likely won't be happy though as he is a great manipulator and has sexually exploited at least 2 previous partners.
I am early 30s, earn more than both of them put together and think I'm probably in the top 10% of earners for my age. I left home 13 years ago, own a property and had a very stable home life growing up. I had started calling out his behaviour in an attempt to make him take accountability before he started grooming her.
I didn't realise the full extent of his behaviour and the effect on me until I was months out of the relationship.
So he probably thinks he has traded up because there is a natural power imbalance and he will have more control.
Sho probably thinks she's hit the big time having this great older man, but the cracks will be starting to show now if they hadn't already.
I feel sorry for her as she has no idea what she is getting into.
Yes I felt betrayed, but also overwhelmingly glad now that he did what he did.
I think every single case is different and we can't generalise.