It's hard to know where to start. You say so many things.
But sales =/= talent. The more money you make off of something, the more talented you are is a very, very naive concept.
Very few people believe that. But almost as naive is the idea that if an artist appeals to a large number of people, then by definition he or she must therefore not be talented -- because the vast majority of people cannot appreciate talent, and don't like it.
That's what I hear every day from writers and bands who haven't managed to hit the big time. There are people with talent who appeal to a wide audience, and people with talent who only appeal to a narrow range. I'm a jazz musician. There are artists I revere who most people have never heard of, and wouldn't much care for if they did. That doesn't mean that the most popular band of all time, The Beatles, has no talent.
But people don't really like talent. I've seen them go crazy over the guitar work by Led Zepplin, and think Gorguts - Obscura is terrible, which absolutely baffles me. I personally know a nine year old, a thirteen year old, and a 10 ten year old who can nearly flawlessly cover Zepplin, but I also know guitarists who have trained for 10 years+ who can't cover a single riff by Gorguts.
And yes, technicality alone does not make good music, I know.
This seems more like personal taste than recognition of talent. Imo, Gorguts is a technically proficient band, whose appeal is specific, and relatively limited. I don't particularly care for them myself. It's a matter of taste. Led Zepplin was an immensely talented band who also gained great commercial success. There's a reason they did, and it's not because people are stupid and don't recognize true talent when they hear it.
Talent, to me, refers to technical skill.
Those are two different things, imo. Technical skill is the tool one uses to free one's innate talent. Without talent, the best you can hope for is competence. (Although it's true that minor talent plus a lot of hard work can produce better work that great talent residing in a lazy personalty.)
Anyone can become technically competent -- although technical brilliance is a gift. But not everyone can produce top level music or books, no matter their technical skills. One also needs talent.
Looking at it from a percentage standpoint, I'd wager 1.5% (this is an actual calculation approximately 125 of music aficionados came up with) of musicians are truly talented and try to further their skill with every album and song.
That may be, (though I doubt it) but I'm pretty sure I could come up with 125 aficionados, including myself, who would have entirely different assessments as to which musicians are truly talented. You really seem to be conflating your own personal tastes with an objective assessment of talent -- which is a fool's errand in the first place.
And to return to King, he has some real technical ability, regardless of what one thinks of his work. For example,
It. A long novel written from multiple viewpoints, told through the lens of multiple characters at different points in their lives, both as children and adults, bouncing back and forth between past and present.
To be able to even make such a book comprehensible, let alone readable, is an impressive technical achievement. I've published six novels, and I couldn't pull something like that off -- I wouldn't even try.
That fact that so many people love his work is a testament to a different sort of talent, the ability to engage a reader. It's not because people are dumb and he's dumbed down his writing to appeal to them.