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In defense of John's, whether you like his writing or not, the man is getting to write and shape the futures of his favorite characters. Maybe he hasn't attempted to branch out because he genuinely enjoys the work he is doing at DC. I don't see any reason to criticize him or really any of the other people on your list for not branching out from the Big 2.
I disagree. There are a lot of industry legends who suffered greatly in their later years because they didn't own anything that they created. They got to write and shape those characters too, but when they got sick, doing so didn't pay their medical bills. With today's creators, you can't find very many major writers who haven't learned that lesson. No one will argue that people like Kurt Busiek, Peter David and Mark Waid don't enjoy working for the big two and never considered it a lifelong dream to work in those universes. But like nearly every other writer, they've done their own work to take care of themselves, to take care of their families and even to stretch their creative muscles beyond the bubble of the big two. Geoff Johns is nearly alone in regards to not doing those things. I think that is more than valid to look at him and ask whether he's too stupid to understand the need to own something yourself, or simply not creative enough to craft something on his own.