Let's talk about a few Action Anime that erased the original premise

--> YuYu Hakusho

It's hard to have this conversation without YuYu Hakusho immediately coming to mind, so YuYu Hakusho was basically a mystery of the week Manga, Yusuke Urameshi was basically a young man with the ability to interact with Yokai, he realized that Yokai needed private detectives and there was great incentive to actually do what these Yokai were telling him, simple enough right? Let's fast forward to the Three Kings Arc, there are Three Demon Kings in the Demon Realm and Yusuke Urameshi has to succeed one of them, who is also his Ancestor, which is the reason he had the ability to interact with Yokai all along, holy premise erasure Batman

--> Bleach

If YuYu Hakusho was a mystery of the week Manga; Bleach started off as a slightly askew monster of the week Manga, Noriyaki Kubo actually wanted to incorporate mystery solving into the story and it was blatantly obvious he was inspired by Yoshihiro Togashi but he gave up after the first few Chapters and just made it straight up monster slaying with little to no mystery solving, so the narrative focused on how Ichigo was balancing his real life with his alternative life as a monster slayer, he had the mod soul - Kon, which works for humans but was made by the Shinigami which doesn't make any sense but then again, absolutely nothing about Bleach makes sense, it will fall apart when inspected, that's how it is in the Action Manga Industry, it's the problem described as building the plane while the plane is already airborne, so there was Don Kanonji who was this pseudo celebrity who could also interact with the Hollow and Noriyaki Kubo threw in Scooby Doo in there too, Ichigo ended up having a full mystery solving/ hollow slaying team with Orihime Inōe filling in for Daphne, Uryu Ishida filling in for Shaggy and Yasutora Sado filling in for Fred, so fast forward to the Soul Society Arc and boom, the original premise is completely gone, Ichigo is actually a Human Shinigami and sadly none of his Friends are but then again, when were they actually ever given equal importance in the first place

--> Naruto - Boruto

Naruto was about the missions, the loadouts, the payments, it was actually cold and gritty if anything, packing a demon wind shuriken instead of a med kit in your backpack was a crucial choice, fast forward to Boruto, none of that stuff matters and you're fighting cosmic aliens, even the Chakra based magic system described in the beginning of the Series got severely retconned

--> JJK - Modulo

What happened to JJK is very easy to explain if you have the context of Naruto, so JJK was about Cursed Energy or JuRyoku instead of Chakra and it was about fighting Cursed Beings and other Jujutsu Sorcerers, so fast forward to Modulo and basically Gege Akutami pulled a Masashi Kishimoto, none of the stuff matters, everything inclusive of the Magic System got hard retconned and we're fighting cosmic aliens again, fuck you