I'm glad we didn't start the story with Luke just running a new Jedi Academy. At best, it's going to be Harry Potter redux, at worst it would be like those terrible Jedi Academy kids books with Han and Leia's kids. Either that, or another youngling massacre, because that's something SW always needs.
Besides, Luke restarting the Jedi - and getting it right - feels like a big enough development that it ought to be shown onscreen, rather than happen between the films. Maybe it's a point that would have worked better if these sequels were coming out five years after the OT, but it's something I'd rather see him do than just hear about in the backstory: "Oh, yeah, there's a whole bunch of Jedi now, it's like they're coming off the factory belt! Plus, Luke did away with all those shitty rules the PT Jedi had, it's totes cool." And if they're focusing on Rey, it makes sense for her to be the only one we see Luke training at this time. More than a cliffhanger, that's what the last scene really is: the true beginning of the new Jedi.
Another thing I've heard rumored that Johnson requested JJ change was that originally the last scene had a slightly more definitive resolution (e.g. Luke taking the lightsaber - they also apparently filmed, or at least scripted, a version where he gave Rey a hug), but that Johnson asked it be more ambiguous.
Anyways, I look forward to the new Rogue One trailer coming out on Tuesday, when we will talk about that film for approximately an hour after which this thread will get back to, I dunno, arguing about whether what TFA really needed was the revelation that Snoke is that weird droid that helped Padme give birth in ROTS.