That sounded a lot more condescending than i intended. I think the most common theory is the wall was built with magic therefore the WW couldn't cross. That problem was inadvertently solved by Bran coming back. The three eyed raven established that once the NK has marked you there's nowhere he can't follow you. It's how he was able to get in to the cave, marking Bran rendered the magic useless.
I can't imagine they mentioned that if they weren't going to set it up potentially for later. Now all they appear to have done is put the blame again inadvertently on Jon's shoulders, it's something I'm sure he'll feel responsible for when he sees the wall and the ice dragon. It's just another example and in my mind one of the most prominent ones in how the show has shifted from intricate nuanced dialogue to big ' Hollywood' moments. I think the actors admitted as much before s7, I remember Kit remarking when he saw the script he knew that it was definitely a different show now completely different to what it was before.