I understand that

In an all out war I expect a stomp for Shanks

Given the gap in their forces that makes perfect sense

In a 1v1 though? Hell no

An effortless one shot just isn't satisfying

Even Kaido wouldn't be able to do that to Kid at this point in the story

Why can Shanks?
I really doubt he's effortlessly did it if Kid is down, maybe he used one of his strongest move. Swordsmen do not need many hit to bring down their opponents.
If Shanks could just effortlessly beat Kid it would make the gap between Shanks and the other emperors retarded considering BM vs Kid & Law and Law putting work against BB
Because he got a cult
Most fans think he is a god...that's why they call him Goda

Sales doesn't prove your story is great
Huh ? Three generation changed during this time . It has been nearly 15 years after time skip . Without atteacting young generations, how can one piece be at top 3 ? That means if one piece bad , all bew generation mangas are terrible.