I honestly think it went something like this. Lucas heard that Disney wanted to make new movies after the purchase of Lucasfilm, and hastily came up with story "treatments" as an afterthought. He then tells Kathleen Kennedy that these story treatments represent his "original vision" for the franchise that he had in '76. Kennedy's thinking "well, I don't know, let's see if we can get someone to make sense of this," and hires Michael Ardnt. He writes a sort of unfinished draft that's fine but the story kind of sucks. JJ reads the script and passes. Kennedy's like "fuck it" and tells JJ he can do whatever he wants. JJ hires Lawrence Kasdan, they break the story for the film and outline the next two (with likely some similarities to the Lucas treatments but probably just general stuff that's similar by chance) with Rian Johnson (I honestly think JJ started from scratch, more or less, which you might think is a hard thing to do given how little time they had, but what Star Wars fan doesn't have an idea about where things go after Episode VI?). Then the trilogy gets made and everyone's happy and the Star Wars saga is declared, officially, to be solely consisted of Episodes IV-IX.
Well that last part is wishful thinking. But I think the other stuff is mostly true. Apparently the Lucas drafts were teen-centric, and probably just had Han, Leia, and Luke's kids as the main characters. I like that JJ has expanded things and with a genuinely new bunch of characters, only one of which may or may not be an offspring of the OT's big three. Also, it appears the early rumors about Episode VII being driven by the big three are unsubstantiated, based on the prominence of what appears to be the new big three (Finn, Rey, Dameron). If they weren't the main characters, why would the trailers and Celebration panel focus on them, when the OT characters are obviously more marketable?
Long story short, I think JJ and co. made the right choice from what I know, but obviously I don't know a lot because they haven't really told us anything and the movies aren't out yet. I'd love a comprehensive behind the scenes doc about the new trilogy starting with the Disney deal that really lets us know how things when down. Unfortunately we probably won't have one of those or know what Lucas's treatments were like until after Episode IX because of spoilers and such.