IMHO, over the course of decades, he's continued trying to reach the lowest common denominator to sell books. As the American public has become decreasingly literate, King has continued to try to sell books (and screen plays) by writing to the bread, circuses and shiny object crowd.
As much as I dislike his politics, he's certainly a capable story teller, and I think any aspiring writer would do well to read his On Writing, in which he openly admits to writing "schlock" to pay his bills, and indulging his more serious pursuits elsewhere, often under pseudonym.
It will be interesting to see how history regards him. Much of his horror is immediate and strikes a chord by taking a common place object, device or situation and making it dangerous (i.e. "Cell", "Christine," "Cujo" etc.) When those objects or situations grow obsolete, the horror will most likely lose its effect. On the other hand, I think his Dark Tower books will have a lot of staying power...JMHO
Your point well made - King writes the same book over and over again.
As far as setting up a literacy foundation, as someone suggested above, literate readers would not stoop down to read the kind of penny dreadfuls that King produces.