When Ariana Grande’s “Thank U, Next” music video premiered last week on YouTube, 829,000 people watched it simultaneously and posted over 500,000 chat messages during its debut. That’s bigger news than how within 24 hours the video was seen by 50 million people. Why?  Because in a world on demand, where you can see anything on your own schedule, having an audience tune into something at an appointed time almost never happens, unless its live sports, award shows or the final season of Game of Thrones. It’s also remarkable that in our modern era — ruled by individual tastes, niches, sub-genres and all that the “long tail” model of marketing documented — there can still be an artist who reaches a mass audience like this. It harkens back to days of yore (which itself is a phrase that should go back to yore), to the time of the monoculture, when there were musicians,…