One measurement put El Capitan's uptake at just 75% of its forerunner, 2014's Yosemite, according to statistics from U.S.-based analytics vendor Net Applications.
In the 153 days between its Sept. 30, 2015 launch and Feb. 29, 2016, El Capitan's user share of all OS X editions grew to 47.9%, or approximately 0.31 percentage points per day. On a per-day basis -- from its Oct. 16, 2014, debut until Feb. 28, 2015 -- Yosemite accumulated an average of 0.4 percentage points of growth.
Even 2013's OS X 10.9, better known as Mavericks -- and the first upgrade that Apple offered free of charge -- was more quickly adopted on an average daily basis than El Capitan: During the 130 days between its release and the end of February 2014, Mavericks grew by 0.38 percentage points per day.
Like Mavericks and Yosemite, El Capitan was made available to Mac owners for free.
It's unclear whether the disparity signaled a broader resistance to annual upgrades, or is specific to El Capitan.
The result is less muddy, however. If more Mac owners decline to upgrade OS X than in the past, Apple's personal computer ecosystem becomes more fragmented -- a distribution of users across more editions -- making it more difficult for developers to justify adopting new features and APIs (application programming interfaces) in the latest version.
A more fragmented world of Macs could also put more users at risk, as Apple supports only the current version, and the previous two editions, with security updates. In 2013, Apple added a third edition to its earlier current-and-predecessor scheme, supporting not only "n," or the latest version and "n-1," but also "n-2." If uptake stalls, Apple might be forced to expand support to "n-3."
As of February, 12.2% of all Macs were running an unsupported version of OS X, or about one in every eight machines. That was very close to the 12.6% of all Windows PCs that last month ran an outdated version of Microsoft's operating system, according to Net Applications' data.
The free El Capitan upgrade can be obtained from Apple's Mac App Store, and supports iMacs as old as mid-2007, MacBook Pro laptops from late 2007 on and MacBook Air laptops from late 2008 going forward.