In a "twist" we all saw coming, the master schemer was reduced to a bumbling idiot—and the conflict between Stark sisters served no greater purpose.
FYI This post contains spoilers
The Season 7 finale, "The Wolf and the Dragon" gave viewers many fake-outs: Will the Mountain kill Tyrion? Will the Mountain kill Jaime? Will Cersei fight alongside Daenerys? Will Sansa have Arya executed? The answer to these questions is no, and when faced with all these possibilities, showrunners chose to kill Littlefinger. While I've been team "I Can't Wait to See Littlefinger Die" for some time, Littlefinger's death felt like a hollow "twist" we all saw coming—and the undoing of Sansa and Arya's character-building with an illogical conflict served no greater purpose than misdirection.
Let's start with the good: We were all happy to see Littlefinger die. I enjoyed that he was killed for confessing his machinations to one of his victims and then waiting to see what she did once she had power. And I also think it was artful to have his death be a direct callback to Ned's arrest and Catelyn's murder: all three believed they were surrounded by allies but got knives to the throat.
While the end result was nice, it feels clear that showrunners conceptualized this scene first, and worked backwards to make it happen without much consideration of its believability or impact. This is similar to Jon's plan to go North of the Wall to capture a wight; there were other ways for him to convince Cersei (or to fail at it), but this one yielded the coolest confrontations. And while both of these scenes felt high-stakes at the time, they were ultimately low-cost, with only Thoros and Littlefinger dying. The problem is that showrunners wanted a scene that was worthy of reaction videos but made Littlefinger's death extremely low-stakes in the process. The show's best twists were dependent on characters' decisions coming back to bite them in the ass (the Red Wedding, Ned's betrayal), but in this case, there was no set-up. The conflict at Winterfell was based on what? A letter that Sansa had rationally explained away?
The Winterfell plot suggests two possible options: That Sansa and Arya were manipulated almost all along and only conspired with each other right before confronting Littlefinger—which would have been bad because then their poorly written, out-of-character conflict was real rather than part of a ploy. Alternatively, we have to believe that Sansa and Arya were planning together all along. In the second case, the show failed spectacularly at communicating that to the audience, or else intentionally kept it from us for a "twist" that wasn't particularly convincing or interesting.
We already know that Sansa and Arya's fight doesn't align with their characters. Many justify the poorly conceived and executed conflict with the fact that it was part of a scheme, but the plot's potency relied entirely on the fact that we couldn't tell which interactions between Arya and Sansa were manufactured. Rather than creating intrigue, this decision cheapened all of the scenes.
A fight between the Stark sisters isn't a fundamentally bad idea for a conflict, especially since they might distrust each other given their brutal pasts and already rocky relationship, but this twist rang hollow because viewers would never believe that Arya and Sansa would kill each other. It's helpful to compare this to another, more successful twist in Season 3, in which Dany burns Master Kraznys and all the masters of Astapor. It's similarly built up to for many episodes, but made thrilling because we believe it's possible that Dany might sell a dragon.
Alternatively, it could have been more suspenseful and clever to let viewers in on the scheme, either by showing spies listening to Arya and Sansa's conversations, or with hints like Margaery's surreptitious plea for her grandmother Olenna to leave the capital while she was under the watch of the Faith Militant. If the Stark siblings were collaborating, we'd lose the "gotcha" moment, but none of the suspense: Would they be able to successfully manipulate Littlefinger? How would they explain to the Lords of the North and the Vale? Would they reveal Bran's power? Who would outsmart who?
Instead, Littlefinger was downgraded from a political and criminal mastermind to a bumbling horndog, burned by the simplest plan all because Bran watched Season 1. Not only were his own plans and intentions not well developed, but it should have taken brilliant strategy to defeat the most mischievous schemer in Westeros.
Why didn't Littlefinger force the issues he cares about by giving Sansa a true dilemma? He could tell her that either she has to marry him or she has to force Arya to marry Robin Arryn—in order to pay the debt of winning the North in Season 6 and maintain the support of the knights of the Vale. Arya would face the real consequences of choosing to give up her life of killing to return to family in a very political world, and Sansa would agonize and maybe even consider compromising to save her sister. Plus, Littlefinger would have done something in reaction to Bran's veiled "Chaos is a ladder" threat. After hearing a phrase he had uttered in private to Varys, Littlefinger would have assumed that the Master of Whispers leaked information to the Starks (also logical since he knows Jon and Varys are aligned with Dany).
Although it's oddly wicked that the master of lies is defeated by an omniscient psychic boy whose father Littlefinger had betrayed, it is frustrating that there is a vague suggestion of scheming by the Stark children that we never verify in detail onscreen or offscreen, unlike all other good schemes in the show (Tywin confessing the Red Wedding, Tyrion confessing to his gambit that exposed Pycelle's loyalty to the queen, Littlefinger and Olenna confessing to the Purple Wedding, etc).
How did the Stark children react when they learned the details behind their father's death and, ultimately, their family's demise? We'll never know, because it was more important to make clickbait TV.