I’d say that actually Luke the character drove the entire plot of The Force Awakens; rather, it was Hamill Himself that was left out.

Why? Because J.J. knew Ford had only signed on for one movie, whereas (they believed) they had Fisher and Hamill for the whole trilogy – at least. If Han Solo’s death was going to have the impact they wanted they needed to bring his character to the forefront. I don’t think it’s a coincidence that we’re hearing the original plan was to have Han prominent in VII, Luke in VIII and Leia in IX. With all new characters taking the franchise forward, the old favourites had to wait their turn.

Incidentally, I bet it actually killed J.J. to direct a Star Wars movie and not have Luke in it.