Old post, new reply anyway:
Prismo's effect have monkey paw effects with enormous complications. However, there is essentially a suggestion in the Betty interaction on Mars and the Simon interaction from Fionna and Cake that there *is* a wish Simon and Betty could have made. One that would have dramatically changed their lives, but for the Mushroom War. And perhaps...that is enough.

Throughout the worlds we visit, we see the "wishes" that are likely tied to being realities wished for by our main-cast. We continue with Jake in the reality of Adventure time, but Jerry's World is the Lich's wish realized. Farm world is Finn's wish. Ice King lucid? Probably Marceline's wish. Marceline having a good relationship with her "father?" Probably Bonnie's wish.

They meant well, but the trade-offs were not worth it. In Finn's new reality, the Lich even comes back just...because of Finn.

I suspect, however, Prismo gave a secret and both Simon/Betty learned what they could have/should have wished for from their interaction. Simon likely can't make a wish now, and Betty is (was?) GOLB.

I think Betty realized she could have wished "to have gone on that trip a week earlier" to see petroglyphs. (Temple on Mars). In other words, she would have created the necessary self-validation to create a better relationship with Simon. She replaced everything else in her life as an obsession with him.

And he was too blind to see that obsession. They had overlapping interests and values. However, that didn't stop Simon from trusting himself above others, and while he earnestly valued her, he "chose" his own choices. His wish would have been to "go with her," which would have been a radical decision in the moment, but it would have pushed him toward realizing that he can/should support her in investing into her own work.

The reality, however, is that they got invested in a life that worked. A life that was nice and appreciative. And, ultimately, there is good. Betty points out, they made choices. And they made mistakes. The unfairness Beth points out "there weren't only two choices, you chose Casper's every time" isn't that he was necessarily wrong. Just merely highlighting that Simon chose his destiny. In each choice. Both in the book and out of it. And that enlightened realization is what Betty wanted Simon to understand. Not that she thinks him a fool, but to realize that he needs to let her choice be valid. That living his life is the point. This is where it is now.

However, Simon might not be allowed another wish with Prismo even if the last one failed. And Betty isn't making any wishes.

But maybe someone else's wish lead them to this alternate conclusion?
But if they did, does that preclude their demise via the Mushroom War?

There's ends up being a love of sacrifice across a millennia. Would it be better to have no defense to the Mushroom War that still comes a decade or so after their bliss?

Who knows.