This is nowhere near a valid argument! First of all, there is no such thing as a perfect technique because every jutsu has a weakness! Second, different things take different times to master. Therefore some stuff is just a matter of thinking things up on the fly while others require years of hard work! How hard does it look to combine kamui and raikiri, two jutsus that Kakashi already had in the first place?! It is not rocket science, so it shouldn't take years! You people should stop acting like every little thing takes years to master!
For goodness sake, are you going to say that making a ham sandwich should take years next?
Finally, they don't have years to train. Kaguya came now! Villains aren't just going to say "I will let you train for some years to defeat me".
The only thing that is nowhere near valid is your own argument. When I said "the manga has established that you need to work to perfect a technique", I didn't think there was any room for confusion on what I meant. This manga has made it fact from chapter 1 that in order to do ANYTHING JUTSU RELATED you need to practice it. I did not mean that any jutsu shown is perfect. No jutsu is perfect.
Different things may take shorter or longer times to master than others, but that does not mean by any stretch of the phrase that you can master a jutsu the first time you do it. Minato took [I believe it was] 3 years to "master" the Rasengan. Naruto took 3 days, and then 3 years beyond that to evolve the Rasengan even further. Not even Sasuke, a prodigy with the Sharingan, can learn something that quickly. He learned the Chidori from Kakashi, but it took him 3 years to master it the way he does now. How hard does it look to combine Kamui and Raikiri? It looks impossible to me, considering Kamui is Space/Time tech and Raikiri is chakra manipulation. Naruto had to train his ass off to learn the basic form of the RasenShuriken, which is the same learning curve as what is needed to pull off a "Kamui Raikiri". A learning curve I'd like to add that Kakashi himself admitted he could never do [which just adds to the ass pull list, by the way].
Your logic about Kaguya also makes no sense at all. You're confusing plot progression with story creation. When you tell a story, theres an unwritten rule about the story teller making sure to follow any rules that HE/SHE CREATES ABOUT HIS OWN STORY. Breaking this unspoken rule has a term of its own: its called a plothole. The term that people have adopted for this particular story has the same effect, but a different name: its called an ass pull. Kishi created a rule that says, "characters must work to achieve something". That rule was more like a theme, as that was what Naruto was all about. Working your ass off to achieve something. Now he's breaking that rule by handing out power ups to characters that have no business with them in the first place. Thats called a plothole, also known as an ass pull. Kaguya's presence changes none of that, because her introduction falls under PLOT PROGRESSION not STORY CREATION. I don't mind Kaguya being introduced into the story. Its the road that lead to it I have a problem with.
If I told you a story about a kid who never did anything immoral in his life. Was the boy scout of his generation; the captain america of his age group. Little Johnny then took a test in school one day. PLOT PROGRESSION is little Johnny failing the test. Its a variable that, regardless of whether he passes or fails, cannot be changed. STORY CREATION is little Johnny never doing an immoral thing in his life, yet the reason why he failed the test is because he cheated on it. Make sense?
And PS: While it may not take years to make your next ham sandwich, never forget that the "professional" is the man behind the counter at your favorite restaurant that spend years learning how to make one.
