Apple owns a stake in ARM and likely has more influence on their direction than they would with Intel. Apple is fond of controlling their destiny and that means controlling the entire stack. They might keep iOS and OSX separate but I could definitely see them unifying hardware across all their lines as much as possible. They avoid the margin paid to Intel and can lower costs via scale.
e: I know they had a stake in ARM at one point, but I'm unsure if they still do.
24 hour battery life (instead of 12) and half the performance in a MacBook Air seems hardly worth it to me. To my mind that device is already more constrained by performance than battery life.
(Conversely I also don’t see ARM/Apple getting too slow to keep pace with Intel on the mobile side, so I don’t see them switching there to Intel, either.)