Watching this spectacle, you had to wonder: When will this happen again? When will another pop culture figure mean so much to so many that people are moved to assemble, hug and dance?

This is a tribute, of course, to Jackson’s singular gifts—his voice, songwriting talent, physical grace, and the list goes on and on. But there is the related matter of historical timing. Fame on the level that Jackson achieved is all but impossible for pop culture heroes today, and quite likely it will never be possible again.

On the most basic level, this is matter of business and math. Michael Jackson has sold an estimated 100 million copies worldwide of his 1982 album Thriller, which spent more than 31 weeks at the top of the Billboard charts. It’s one of those high-water marks that nobody will touch, because record stores are vanishing, and along with them, megahit albums are vanishing too. A big week on the Billboard charts is a quarter-million units sold, which is about the number of units the Jonas Brothers moved last week with their latest release, which opened at No. 1. And it’s rare for an album to last even three weeks at the top.

People who buy music tend these days to buy—or steal it—online, a song at time. But even if nobody achieves album sales on a Jacksonian scale, couldn’t he or she be an artist every bit as popular, every bit as loved, every bit as listened to?

Probably not. The pop-idol field—like every field that can lead to super-fame—is more crowded than it has ever been, and the variety of routes to stardom keeps growing. When the Beatles were on The Ed Sullivan Show in 1964, more than 70 million people watched. Yes, the Beatles were that good. But at the time, there were three networks and the radio. No Facebook, Twitter, video games, movie multiplexes, Sirius radio, malls or a dozen other potential drains on an audience. There weren’t a lot of rock bands, either. George Harrison was the only Beatle who’d visited the United States before the group landed for that historic performance.

Likewise, Michael Jackson had MTV, which was the place for music videos, and as close to an Ed Sullivan platform as he needed. Of course, it’s been a long time since MTV played hour after hour of prime-time videos. Today, you watch music videos on YouTube, but because there are no programmers to curate what you see, every artist has to compete with thousands of others. And now that anyone with a computer has a miniature studio, and anyone with an Internet connection can post a song, there are more genres, subgenres and artists than ever.

That’s why even Michael Jackson would have a hard time becoming Michael Jackson these days. Come to think of it, Farrah Fawcett, who also passed away this week, would never have become Farrah Fawcett if she showed up in that red, one-piece bathing suit today. In the ’70s, she became the fantasy of choice for every post-pubescent teenage boy in the country, selling 10 million posters of her iconic, high-beam smile. Now, there are so many vixens grinning seductively from so many Web sites and lad mags that no single woman could ever commandeer the public imagination in quite the same way. There is no “this year’s model” anymore. There is this week’s model, and that’s about it.There are plenty of upsides to the new unfiltered, multiple-choice pop culture of 2009. We get to decide what we want to listen to and watch, and we can listen and watch whenever we want. But there is something sad about our infinite menu of options. It could very well mean the end of true superstardom and with it, the end of the collective experience on display on Thursday night in Union Square.