PARIS — After another year of plunging music sales, record company executives are starting to contemplate the unthinkable: The digital music business, held out as the future of the industry, may already be as big as it is going to get.

The International Federation of the Phonographic Industry, a trade group based in London, said last week that sales of music in digital form had risen only 6 percent worldwide in 2010, even as the overall music market had shrunk 8 percent or 9 percent, extending a decade-long decline.

In each of the past two years, the rate of increase in digital revenue has approximately halved. If that trend continues, digital sales could top out at less than $5 billion this year, about a third of the overall music market but many billions of dollars short of the amount needed to replace long-gone sales of compact discs.

“Music’s first digital decade is behind us and what do we have?” said Mark Mulligan, an analyst at Forrester Research. “Not a lot of progress.”