It's OK! We've pre-grieved!
As with so many aspects of 'Succession', you're set up for things to go horribly wrong and are still shocked when it happens.
At no point in any part of 'Succession' are we unaware that these are despicable, awful people who are bound together only by their unholy quest for power or how unhappy they all are. Should we have expected anything less than this? In a word, no.
In the ninety-odd minutes of the finale, 'Succession' pulled together all remaining threads in the show and wove them together in such a way that is truly a marvel of storytelling craft. There isn't anything left to ponder about, because everything is where it should be. Let's start with the least present character in this episode and work our way towards - Matsson. Even though he shares a pivotal scene with Tom where he basically asks if he can cuck him, Matsson doesn't really get up to much here except for roaring his underlings to life and querying Shiv on a few points. It makes sense because there's an inevitability to this all that we don't necessarily need to follow Matsson's story or efforts. After all, it's the Roy show, not the Matsson show.
Tom, likewise, is kind of on the periphery although he ends up (seemingly) on top. When he prostrates himself before Matsson in that restaurant scene, you get the horrible sense that Tom would actually knife anyone and anything to get his way, and as Shiv so eloquently pointed out, he'll "suck the biggest cock in the room" to get his way. In fact, Logan would probably approve of everything Tom had done up to this point. It makes complete sense that Tom would be in charge because he's an empty suit. He won't make any changes, he'll do what Matsson tells him to do, he'll hold fast on ATN's current strategy because it's working for them even if it's utterly reprehensible, and he'll gladly assist Matsson in carving up Waystar Royco for stripped parts because, well, he's not attached to any of it. The only thing he cares about is power. This isn't his family's legacy, he's just a "highly interchangeable modular part", as both Shiv sees him and no doubt as Matsson sees him. That's why he wants him for this.
It's a real credit to Kieran Culkin's ability that he is able to play Roman with such aching vulnerability. When we see him in Barbados, he's wearing something akin to a child's T-shirt and looking nowhere close to the slick operator he once was. The fact that he went to Barbados to meet his mother, who really is a terrible mother by the way, speaks to how desperate he is for someone to look after him because he's so incapable of doing it himself. They're all such warped homunculi of Logan Roy, but Roman is by the end the only one who seems to see himself for what he is and what they all are - which is probably why Kendall tried to blind him, in a lovely little homage to 'King Lear'.
Kendall has always been a pathetic creature, internalising all of his father's cruelty and rage in order to become him, but his roaring "I'm the eldest boy!" at his sister evoked the kind of petulance that Logan talked about before. These are not serious people. Roman knew this about himself. Shiv knew this about Kendall, but Kendall refused to see any of it - and lashed out at anyone who tried to tell him otherwise. From the very opening scene, we're told that he doesn't have the numbers, yet Kendall believes that he can will them into existence. When it's all over, and his failure is so complete, how many of us expected to see a shape fly past the window when Matsson signed the contract?
Kendall couldn't bring himself to do that, obviously, because so much of him is a move - designed to evoke responses favourable to his goals. He tells everyone to vote his way to do it for Dad. When he begs Shiv to vote for him in the side room, he tries to rope in her pregnancy and her future as a mother. In the end, there was nothing underneath it all. It was just empty, aimless grabbing that had no soul and no compunction about it. Shiv, in the end, saw it and had to stop it because how could she live with herself if she didn't?
There's no way that Shiv planned this all along, or that she wanted Tom to get the throne so that she could pull the strings from behind. Rather, Shiv made the choice to stop Kendall rather than let Tom and Matsson win. It's particularly galling when, for the entire series, we've seen her as someone with agency and a head on her shoulders fully reduced to a wife and mother. He's not Mr. Shiv Roy, she's Mrs. Tom Wambgans now, for better or for worse. Probably worse.
Ultimately, 'Succession' in these final episodes has been about American decline. Like 'The Godfather' and 'The Godfather Part II', we're seeing an American dynasty tear itself apart through infighting and backstabbing for power, and in the end, it's all washed away into nothingness. Michael Corleone, in the final moments of 'The Godfather Part II', is an older man and alone except for the ghosts of his evils. Kendall Roy, in the final moments of 'Succession', is a man stalked by his failure and by the memories of his father - literally, in that Colin, his dad's best friend and bodyguard, is with him. Waystar Royco will, eventually, be ripped up and sold for parts just like Matsson promised and nothing will be left of them - except for the billions and billions of dollars they all just made. But when has 'Succession' ever been about money?