The Expert Beginner has nowhere to go because progression requires an understanding that he has a lot of work to do, and that is not a readily available conclusion.
First, some background. The author's conceit is that some classes of people who learn a certain practice get stuck at a phase he calls the "Expert Beginner", where they reach a kind of plateau of functional competence but deprive themselves of the outside feedback needed to progress any further, and thus founder in a swamp of self-delusion about their own skills.
His large-scale analogy for discussin the topic is bowling. Mine, as you might guess, is writing.
I notice a similar effect with a great many self-published or marginally-published authors. They reach a certain plateau of functional competence (and sometimes not even that), and then stall and go no further. Work produced years down that road is indistinguishable from work produced when they first started down it, except in the most superficial ways. As the author of the piece puts it, what they have is not ten years of experience, but the same year of experience repeated ten times.
This is a terrible place for anyone to get stuck, but it's doubly destructive for an artist. An artist who hits a comfortable plateau and simply does what he knows will work is not creating anymore, but simply laying bricks and filling in time.
My original theory about why this kind of plateauing happens involved a simple case of aversion. You don't want to endure the pain of continued growth, so once you find a niche in which you can repeat yourself, you get stuck there. You stop looking for things to give you badly-needed kicks in the ass — e.g., the guidance of others, especially those who are, oh, not self-published and might have a thing or two to tell you about how to market your work to people who might actually want to buy it.
It's tempting to believe self-publishing is a self-limiting and closed-ended enterprise, where the only smart thing for the really ambitious to do is find a real publisher. I get close to believing that, but I always pull back from it — although I leave it as an open-ended question whether I hesitate to commit entirely to such a view out of some residual self-serving prejudice about the whole affair. I'd like to think the distribution problem can be solved without resorting to tons of money, but so far the results haven't been encouraging.
Anyway — there's more than one way to get stuck in such rut, or on such a plateau, but my point is that the most common way is to stop seeking outside challenges. Or to not consider them worthy of being sought out in the first place.