Littlefinger's squanders his potential for winning the throne
Beginning in Season 5, as his infatuation with Sansa grew, Littlefinger started to make uncharacteristically bad decisions. The first was to wed Sansa off to Ramsey Bolton after the Boltons take Winterfell. Even when he tries to warn Ramsey off hurting Sansa, the implication was either he doesn’t know how much of a monster that Ramsey is or he didn’t care enough to think the bastard’s cruelty would extend to his bride. Either option clashed with our assumptions of Littlefinger's vast spy network and his love for Sansa Stark.
In that same season, Baelish went South to broker a vague deal with Cersei: she wanted information about Ser Loras Tyrell visiting Littlefinger’s brothels, but he wanted the title of Warden of the North because Roose Bolton dared to marry Sansa off instead of returning her to King’s Landing. Littlefinger eventually rode with the Knights of the Vale to the Battle of the Bastards and remained content with living by Sansa's side. He never claimed his title of Warden of the North. Instead he expressed interest in the Iron Throne, where Sansa could be his Queen, an ambition that makes his previous maneuver with Cersei absolutely meaningless.
As the television adaptation abbreviated and surpassed the plot of George R.R. Martin's novels, the show shifted away from medieval politics and sacrificed one of the most efficient political operators. Littlefinger is suddenly cursed with fated decisions and robbed of nuance. In Season 6, Sansa confronted Littlefinger about Ramsey in a scene that bolstered her character and solidified Baelish as a failure: "If you didn’t know you’re an idiot, if you did know you’re my enemy."
Instead of aiming directly at the chink in Littlefinger’s armour by bringing about his downfall because of something he doesn’t know, Season 7 cast off Littlefinger as a necessary consequence of creating tension between Arya and Sansa. Weiss and Benioff admit that they wrote up to the fake Arya accusal in Littlefinger’s final episode, which means he only created conflict between the Stark sisters as much as they were willing to doubt each other.
Scheming to break Jon, Bran, and Arya away from Sansa is a unnecessary move in a season when Sansa knows she needs Littlefinger for his Knights of the Vale. Baelish is unnerved by the other magical Stark siblings, but Bran doesn’t care about his presence otherwise and Arya was easy enough to con into being agitated by the old scroll, therefore she was controllable. We’re expected to believe that the same character who waited years to exact revenge on the male Starks is suddenly so unsure of his position that he must create “chaos” over seven episodes in Winterfell? And for the love of Sansa somehow? The arc of the character doesn’t feel like it should end with blind incompetence.
It wasn’t entirely Littlefinger’s fault that he had to be an idiot for the show to move forward. He started from the very bottom of the social political ladder and was able to climb the chaos ladder to a few rungs from the top. What he wasn’t counting on in the end was that Catelyn’s children would be so magical. Sansa could have had Littlefinger killed last season, but thought she needed to keep him around for the Knights of the Vale. Littlefinger should have stalled for Sansa once again, like he did in Season 3, but he pushes his luck because, in the end, the true threat of Westeros isn't sly politicking but zombies and the world's dragon-incest saviors. We should be happy with the quality Aidan Gillen performance we were gifted, no matter what stupid decisions he was forced to make.
House Baelish is dead. Long live the reputation of the mockingbird.