Something I posted on the general subject of Ditko and Kirby leaving Marvel in the 1960s, and the "end of that era:"
I agree with the general feeling [about how good Kirby was in the sixties], though I'm also a fan of Kirby's NEW GODS and a handful of his other self-scripted works. However, we should keep in mind that Stan Lee probably would have left Kirby and Ditko if they hadn't left him. Stan quit being a regular scripter in the middle 1970s, and I believe his main excuse was that he just had so much to do campaigning for the Marvel Age of Comics, he had no time for writing. There's probably some truth in this, but I think that if he'd really wanted to do it, he'd have found a way. I'd have to check the timing, but at some point in the 1970s he started pulling down a salary as "publisher," to the extent that he could even step down as chief editor, and ceased to have anything to do with the published comics. Though I get the sense that Lee really got a kick out of writing at times, it was a job first and foremost, and when he got a chance to step down and make money doing something else-- something that allowed him to express the persona he'd created as a "god of comics"-- he did it, and barely did any writing after that. It's also possible that he, more than Kirby or Ditko, was getting burned out on regular comics. I like a little bit of his mid-70s work, but even his best stuff is pale next to his lesser sixties work.