I’ve been seeing a lot of people saying Kid’s fate makes no sense after all he’s done so far, but I’ll go the other way and say it was a good plot twist like we haven’t seen in One Piece for a long time.
First of all, it’s worth pointing out that just because a character appears a lot, it doesn’t mean they can’t be killed off, let alone have grander roles awaiting him in the future. For a comparison: in A Song of Ice and Fire, Ned Stark was one of the main characters in the first book, and one who was deeply involved with some key mysteries in the series (such as Jon Snow’s parentage). Then he was killed off in the first book, and all the secrets he knew died with him. A similar path was followed by Robb Stark: throughout three books, we see him rise into the role of King in the North, and as the Lannisters’ main foe he was a pretty important character. That didn’t save him from being killed off before he could achieve any of his goals. Oberyn Martell is introduced halfway through the third book, but he gets a lot of fleshing out and an important role in the story – until he got killed before he could achieve any of his goals.
Does that mean A Song of Ice and Fire is badly written? No, because all those deaths served a narrative purpose. Through Ned, we were introduced to much of the series’ world and politics, and his death set in motion many of the events that would shape the following books. Robb’s actions helped reshape Westeros’ status quo, and his death led to many ramifications that further expanded the story. Oberyn was our introduction to Dorne as well as the Martells (who later become important power players), and his death had pretty big ramifications – such as Tywin’s death and Tyrion’s escape to Essos, where he’s set to join Daenerys. Just as importantly, all those deaths made sense given what had been previously established about those characters. Ned was too honourable and underestimated how cut-throat King’s Landing politics were; that was his undoing. Robb Stark was too honourable too, and that was what led him to commit the series of blunders that led to his death. Oberyn was an overconfident hothead; that caused him to make the single mistake his opponent needed to kill him.
People may think Kid was wasted, but he had relevant roles in the plot already. Law would never have defeated Big Mom by himself, which means Luffy would either have to fight two Yonko (and lose) or defeat Kaido only to face her (and lose as well).
More importantly, Kid’s attack on Shanks allowed Red Hair to obtain a bunch of Road Poneglyphs, thus making him an even bigger contender for the One Piece. Of course, Oda could’ve made it so Shanks already owned all those Road Poneglyphs; but that would’ve arguably made him nonsensically competent – after all, Road Poneglyphs are supposed to be incredibly hard to get by. Even Big Mom and Kaido only owned one each. Now, thanks to Kid’s actions, Shanks is many steps closer to finding Laugh Tale.
And then there’s the fact Kid’s “annihilation” (which I won’t call ‘death’) was hardly expected after he’s been such an active character for so long. It’s a surprise, one that isn’t nonsensical (after all, we’ve known Kid dreamed of taking down Shanks for a very long time, and it’s been established how reckless he is), and which opens new, interesting possibilities for the narrative (namely Shanks getting closer to finding Laugh Tale, perhaps closer than anyone else).
So no, Kid getting rekt now wasn’t “bad writing”, a “waste of character”, or a “betrayal of our expectations (?) for that character”. It was a good twist, one that was but a natural consequence of previously-established narrative elements, and which opens new possibilities for the story.