I'm going to press X to doubt on Jack and Jamie outlasting MoRon. Jack's got kids, and that means there are real world consequences to his life in a way that isn't true for any of the others. He's also got one woman already draining his bank account if he's paying child support, which I'd assume he is.
So what's Jamie's appeal? She's an independent woman. She's got her own income. She's emotionally independent as well; she can hold her own and doesn't need someone to prop her up. She's even out of town a good amount of time with all the Cons she goes to, so there's not a lot of burden to taking her on as a significant other. And hell, you don't even have to marry her. You get free sex, someone who's not very intellectual but is fairly witty and fun-focused, and you don't have to pay out much of anything at all.
That's all about to change. She's already been depreciating her sex appeal for the last few years. Every pound she puts on decreases her desirability as a mate. Now she's about to sink her finances. Jack hasn't shown any indication that he gets anything emotionally out of her 'star' status - he's not in her mentions, it doesn't seem like her fame builds him up at all so she's about to throw away her financial security for something that he gets absolutely nothing out of. If she starts turning to him for payday loans to cover the mortgage while she's forking over the money for her lawyer, Jack is going to resent the hell out of that. At least the other woman he's paying for gave him children. Why should he be on the hook for some chick he hasn't even married and isn't the mother of his kids?
And it's not going to matter how emotionally independent Jamie is - this lawsuit is clearly already taking an emotional toll on her, and that's going to affect Jack and his kids sooner or later; whether she turns to him for a shoulder to lean on or whether she gets snippy and short-tempered with her pretend family the way she gets with people on Twitter. She starts sniping at the kids the way she's sniping at people on Twitter and that's not going to go over well. She may think she can handle it now, but the lawsuit is going to be intense. She starts draining Jack emotionally and financially when the whole point of the relationship is that she's not supposed to be a burden and the whole thing is going to collapse.
Ron, on the other hand, has crippling insecurity. He benefits from Monica's 'star' status because it feeds his ego, he directly engaged in the criminal conduct, so he's got a personal dog in this fight, and he gets to feel dominant by running out to twitter and slaying the trolls for m'lady. Their relationship isn't going to have as much trouble until the moment when Ron realizes he is utterly powerless to control the outcome.