"The software guys always resented the retail distribution and its physical costs," said Baker. "They're used to selling bits and bytes."

During the Monday keynote at Apple's Worldwide Developers Conference (WWDC), Philip Schiller, the company's head of marketing, claimed that the Mac App Store was the world's largest seller of personal computer software, and was beating Best Buy, Wal-Mart and Office Depot in that arena.

But other factors beyond cutting out the middleman were probably more important to Apple.

"This affords them an additional level of control," said Baker, referring to Apple's well-known fondness for holding tight to the reins of business, from hardware and software design to retail with its own chain of stores. "For them, controlling the user experience [of purchasing and upgrading] is more important than the money."

Baker expects that Lion will do well, but cautioned that even in the best circumstances, revenue generated by the upgrade will be "just a footnote" to its total income.