You are making my point by introducing JW. JW was also a hugely nostalgic trip and made a huge chunk itself. So did Age Of Ultron. I could look at the rest of the movies released in 2015 but my point remains.
SW's huge opening and incredible legs had EVERYTHING to do with it being the only ticket in town, the reuniting of the band after 32 years (as I said) and insane hype and great reviews.
There was NOTHING to compete with it in mid December, period. I would gamble it doesn't open to 248 million if Ultron is still 2 weeks into it's run and JW would have definitely cut it's legs when it came out.
TFA, like Avatar and Titanic before it, ran virtually unopposed into January and beyond.