Star Trek became a success in spite of Roddenberry, not because of him. That's true for both TOS and TNG.
He was very much instrumental in the success of the original Star Trek. I don't know how anyone could interpret things any other way. As far as his contributions to TNG? It was pretty successful right out of the gate. Though I'll leave it to those better informed than I about how much of TNG was Roddenberry's and how much of it was Gerrold, Fontana, Justman and others.