There were competent kings and competent Empires.
Indeed, it's tough to decide where the Roman Empire really began it's decline. It's not a singular event but a centuries long decline. Same with the Spanish Empire and English Empire.
Indeed, the English Empire may have collapsed but that's mostly because Britain just got bored of it. There's no traditional collapse for the breakup of the British Empire
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I can think of some dramatic changes as well. The fall of the Tokugawa Shogunate of Japan wasn't due to incompetence, but instead the culture shock of a full iron battleship from USA visiting Japan when they were still a swords and samurai culture. This broke the Japanese trust in the Samurai system and led to a violent revolution resulting in incredible industrialization. But I don't think the Tokugawa Shogunate was ever considered especially corrupt or incompetent.
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Now that being said: Dictators fall into the dictator trap. A bad king who becomes a narcissist and dictator will fall under the pattern you describe. But that doesn't really happen all that often. That's why it's so memorable when it DOES happen