iPod and its siblings revolutionised the music business and gave labels a lifeline just when they needed one. On Apple’s own stats, shown Wednesday, it still has 73.8 percent of the mobile music player market, with “other” manufacturers languishing on 18 percent, SanDisk (NSDQ: SNDK) 7.2 percent and Microsoft’s Zune 1.1 percent.

So the number of people who own at least one iPod is smaller than Apple’s headline stats because some people own more than one? Big deal. A company that successfully forces customers to shell out again after 18 to 24 months is a company that’s doing well; a tactic seen in other sectors like white goods.

But Mulligan, whom we followed up with, has a darker warning for the companies that rely on iPod: “I don’t think there is anything that reflects particularly badly on Apple; this is what device sales are all about,” he says.

“But it is a big big deal for digital music sales. It means that the key dynamic in digital music sales is changing direction. The number of new potential customers arriving in to the marketplace is slowing.”

Mulligan’s thesis, which cuts through the larger numbers being bandied about, would suggest that digital music sales, which still aren’t offsetting record label revenue decline, are set to plateau as does the number of people who own players.

“And, of course,” Mulligan tells us, “if you get more sophisticated with the modeling, you can end up with even lower numbers (e.g. factoring in multiple device ownership etc).”

One possible route to continued growth, we reckon - unlimited-access music services like Spotify and Rhapsody that, unlike the a la carte track download option offered by iTunes and its ilk, offer up the entire “celestial jukebox”. We’ll see…