Well, from a writer's standpoint he is a hack, but an entertaining and successful one at that. Not to detract from him because the dude crafts some compelling stories and ideas. It's just that he sucks at dialogue for the most part, and the overall craft of writing...which really is fine in the end.
For the majority of writers, garnering that type of success doing what you love is worth more than anything else. I think there are plenty of far superior writers that wouldn't mind "giving up" and just going full on formulaic model and churning out the next pablum on an annual basis. (Exactly what Dean R Koontz way, way back when he gave up on trying to be good.

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To be fair, I think part of the thing with King is that he simply enjoys telling the stories he tells, and let's the stories mostly coalesce as they go. Sure, most writers enjoy writing what they write, but King almost takes it to another level. He is fast and loose with his writing, writing volume way beyond many other authors. He found his niche in the market and owns it, but when he slows down I think he can spin a fantastic tale. Case in point, the first half of The Dark Tower series, where there were many years and even decades between releases. I think that was some of his best writing, ever. The Wizard and Glass is probably one of my favorite novels, ever.
But his big piece is that he doesn't plot, or at least, it's vague ideas. He let's the story come to him, which explains why many of his novels of the decades have been tomes, sometimes unnecessarily so. And his biggest challenge is the ending of a story. Rarely makes a first-rate ending, at least, it's not his norm. But I'm fine with that, as I don't often need to seek out award winning material to read, I just want a good ride. And even if an ending lets me down, be it in a book, movie, or TV series, I really don't mind much, so long as the journey was fulfilling. People will take a bad ending and use it to slam the entire body of work, as though the end is the only point of a story that matters. And frankly, for King, it is basically the least important part of a story. The story, for him, is the journey.
And while dialogue is not often his strong suit, his characterizations are more often on point than many contemporary authors, who rather deal in stereotypes and idealized dream characters (often idealized representations of themselves - I'm looking at you, every female author of young adult novels with female heroines!).