CUPERTINO, Calif.—People have been asking for a 15-inch MacBook Air for about as long as the Air has existed. The Air was originally conceived as a laptop that was all about extreme portability. But over time it became the default MacBook for most people—and plenty of users don’t need the performance of a MacBook Pro but might like a little more desktop space or a bigger screen on which to watch streaming TV. Apple has targeted those people here.