Sherlock couldn't let Moriarty go (and couldn't replace him)
Sherlock brought to the screen some of the detective's most important characters from the books, and among them was Jim Moriarty (Andrew Scott), Sherlock Holmes' greatest enemy. Moriarty's presence was teased since Sherlock's first episode, and he made his first appearance in the season 1 finale, "The Great Game."
Moriarty continued to be the main villain of the series in season 2, until he committed suicide in the season finale, "The Fall of Reichenbach", to pressure Sherlock to do the same. Sherlock's version of Moriarty was a charming, manipulative and impulsive genius, whose intellect was on Sherlock's level, making him a dark mirror of the detective, and the loss of him was one of the series' biggest mistakes.
Surely the same thing happens in the books and Moriarty dies while Sherlock fakes his death, but Moriarty's death in Sherlock happened too soon. Sherlock never recovered from Moriarty's death and couldn't let him go, to the point that he teased his return at the end of season 3, but that never paid off. Moriarty briefly returned in a flashback in season 4, and the series tried too hard to find a good replacement. Charles Augustus Magnussen (Lars Mikkelsen) seemed to be the best replacement for Moriarty, but he only appeared in one episode.
The False Explanation For Sherlock's Death Was Set Up For Disappointment
Sherlock season 2 had the most shocking ending when Sherlock, after Moriarty's suicide, had no choice but to jump to his death from the roof of St. Bartholomew's Hospital, with John witnessing the whole thing. John even checked Sherlock's pulse, and thus the Great Detective died... but not really, since just like in the books, he faked his death. How Sherlock faked his death was the biggest question left by Season 2 and gave way to a number of very clever and creative fan theories. Unfortunately, when it came time to explain how Sherlock survived the fall, the series totally failed the audience. sherlock offered different explanations, and Sherlock himself gave one of them only to Anderson, who had become the biggest theorist and fan of Sherlock Holmes.
However, it is unknown if that encounter occurred, as Anderson noted that he would be the last person Sherlock would trust with such information, and the detective disappeared as soon as Anderson realized this. Sherlock never gave an official explanation for how Sherlock faked his death, and many consider this to be the moment the series' downfall began.
Sherlock Had A Bizarre Victorian Drug Trip Special Episode
To make the wait between seasons 3 and 4 more bearable, Sherlock released a special episode titled “The Abominable Bride.” This episode is set in an alternate timeline in Victorian London, but it is revealed that it is all a drug-induced dream of Sherlock's while he was on a plane back to England. The episode goes back and forth between the Victorian dream, the present, and the dreams within the dream, and the series uses Moriarty as a link between the dream cases and real life. The episode is full of references to the books, but they feel forced and like obligatory moments of fanservice. Ultimately, “The Abominable Bride” adds nothing to the main series and further shows how the writers didn't know what to do with Sherlock and company after season 2.
Sherlock's deduction skills became hard to believe
Much of the appeal of BBC Sherlock was how he visually represented his deductions, allowing the audience to follow them and connect the dots. Sherlock's mind works very fast, but literally seeing him deduce things in a matter of seconds helped the audience understand what was going on in his head, making his abilities and his cases believable. After Sherlock season 2, the detective's skills became increasingly difficult to believe, and he even predicted eerily accurate things out of practically nothing. This also left no room for surprises since Sherlock already knew everything about everyone, and every time a "twist" occurred, they were silly and not believable.
