The fact is anyone who can write can write a book. The questions an aspiring author must ask himself are:
Do I have something to say (or, do I have a great story)?
Am I capable of writing it well?
Is my book marketable?
I've read that agents and editors are looking for great stories, well written. They can afford to be picky because of the sheer volume of submissions they receive every freakin' day!
I suspect that the problem is so many people think that writing is the easy path to riches or at least financial independence. I don't know when or how that got started, but it's not; it's hard work and it's hard work that accelerates once you sell the first book. Edits and marketing tasks are added to your workload. Deadlines for those must be met while you are hard at work on the next book.
If I were to hazard a guess at the mindset of Mr. Marquardt, I'd suggest the possibility that he saw the success of Duma Key and asked himself why that was so when Keller's Den was so similar but was not selling. (Of course, those similarities exist only in his mind.) Someone has to be to blame for the lack of success for his book and that someone must be Stephen King. Why Stephen King in particular? Well, because . . . because . . . "he stole my idea!"