Wait what? What is this?From brutally murdering a character which made a female editor cry
I don't know if the Japanese authors just say "fuck it", I think they plan out a plot with the ending and then can't come up with new ideas when the manga is extended.
Toriyama famously was banned from ending Dragonball because it was the most popular manga in the magazine so he just repeated the Frieza arc over and over again with higher and higher powerlevel enemies.
The Death Note author revealed in his next manga, Bakuman, that he wanted to end Death Note with L and Light both killing each other but the editors prevented him and told him to keep the manga going so he just did the Light arc again with Near and missiles and Yakuza and shit. Indeed, that would have been a more natural ending. Light kills L but suprise, L set a trap as a dead man's switch and both characters die and the manga ends.