Apple had some very impressive, record-shattering quarterly earnings numbers to report on Tuesday, but one figure that wasn't so great: iPad sales.

Apple sold more than 21 million iPads during the December quarter — down from 26 million the year before — which, to some, might seem to indicate a further sign of the tablet's steady decline.

See also: 14 Things Thicker Than the iPad Air 2

But Apple CEO Tim Cook seemed optimistic about the future of the iPad, calling it "great business" in the analysts call after the release of the Cupertino company's earnings report.

Cook believes that "over the long arc of time," there's still a "very, very bright future" for the iPad. An improvement in sales numbers isn't going to miraculously happen overnight.

"I'm not projecting something very different next quarter or the next," Cook said. "I'm thinking over the long run."

One interesting note: 50% to 70% of people buying tablets are first-time iPad buyers. But the upgrade cycle is a lot more infrequent than it is with, say, an iPhone. People probably replace an iPad somewhere in between a phone and a desktop computer, he said. The bigger iPhone 6 Plus probably cannibalized some of the interest in the iPad, too.

In a somewhat resonant twist of fate, the earnings report happened to fall on the fifth anniversary of the first announcement of the iPad. But though the numbers continue to be on the decline, Apple is optimistic about the iPad's future.