I think we must look at the full picture. She still gets 2 billion (?) and she doesn't have to watch Kendall ascend the throne. We are lead to believe that Mattson's deal is incredible and that Waystar (on its own) may never truly recover, even with a great CEO. It does make sense to cash in and be done with the family feud for good. It's a rational choice. Kendall could very well freeze her out of all operations and bring the company to the ground. What good would her name and shares do her then?
Kendall wanted control over the company because he was primed for it since he was a kid. It was the whole purpose of his life. Roman only wanted his dad's love and approval. With Logan gone he has no purpose. He only needed closure, which he kind of got in the end. Shiv is more complicated. She's been sidelined and wanted to prove herself from the beginning. And the only way to do so would be to become CEO of Waystar. Kendall took this role for granted and Roman never really wanted to compete with his siblings. The only one truly competing internally, is Shiv. That is why she is so conflicted and could turn on a dime. She absolutely don't want to concede any power, but losing to Kendall cuts her to the very core.
The money isn't an issue. She definitely didn't make that decision to cash in and be done with it. The payout is almost irrelevant. Now Roman, for example, makes perfect sense. He doesn't want the job, he doesn't want the family feuding, and he doesn't want all the bullshit that comes with it. He's done. He's completely broken and checked out. So if we were talking about Roman here, then it would make much more sense. I actually think how it all ended with Roman was perfect.
In fact, you could make the argument that he was the "winner" here. Certainly more than the other two. He's out. His father is gone, and while that fecked him up, he doesn't have to be forever dealing with that toxic relationship. He's dropped his facade, he never wanted the job but couldn't admit it. So now he doesn't need to deal with that bullshit anymore. He's free. He's still a mess, but he's in a better place than the other two. I have no problem with how that arc ended. I think they did an incredible job there.
We are dealing with very capricious people here, no doubt. Shiv making that decision at the end is not
completely out of character. We're not talking about a tonal handbrake turn here, but I just couldn't help but feel that it was slightly forced when it happened. I liked the episode a lot, but this was the one nagging thing that kept playing on my mind.