They can also turn into coke zombies who are still able to write. Stephen King claimed that he was so out of it during that period of his life, he straight up doesn't remember writing Cujo at all[0].
He's undeniably talented, which even his detractors will admit. (The complaint of literary snobs isn't that he's a bad writer, but that he puts his talent toward genres of writing they don't like.) His quality profile is also notably variable, which is also part of why he has so many detractors. He also came up in a time when publishing was more forgiving of idiosyncrasies and personal difficulties. (In today's world, a literary agent will fire and cancel you on Twitter if you get a pronoun wrong by mistake.) Finally, he was always a high-functioning drug addict, and there are a lot more of them than most people think (although it seems rare to stay high-functioning for more than a few years).
The thing about King, love him or hate him, is that he loves to write. Most people, even among successful novelists, don't--and that's OK, because it's extremely difficult to do well, and the rewards are astronomically uncommon. I think this explains why he was able to write decent books even while battling these demons.