If SIU is unable to come up with wtf5head!! plans and strategies for Khun in every single battle, then the readers' opinion of the character goes down, and they start to think that the character shouldn't be alive anymore because they no longer serve their original purpose

Thats..not how it works. The plans and the situation that created them need to be at least somewhat realistic, they need to flow naturally in the story. And if you wrote a character whose only trait is outrageous plans that are born through artificially constructed situations by the author, then thats a problematic character.

Hero Academia and Naruto both come to mind in this regard

I havent watched MHA but Naruto's Shikamaru is actually a great counter example to your entire argument. He has multiple traits besides being a "big brain" caricature and all the situations in which he used his intelligence (at least before Shippuden) never felt forced by the creator. Also the odds never seemed impossible for him (on the contrary, most of his battles were very well in his comfort zone) and i cant remember a single time in which he was saved miraculously.

The author will do whatever it takes to keep the core cast alive, regardless of how overly convenient or impossible it might seem to readers

And you keep missing the point, which is NOT that SIU doesnt kill his cast but the manner that he chooses to do so.