At some point the missiles sites would have been completed and the missiles become operational. The USSR would at some point have wanted to let the Americans know they were there as a bargaining point over U.S. missiles in Turkey, Berlin, or some other issue (or a combination of all). If this had come-up in 1964 then it would have been a Presidential election issue in the US.
But, the missiles were given away by more than just U.S. photo recon. Those planes were tasked in part to verify rumors coming out of the exile community that the Soviets were moving heavy weapons into Cuba. Naval surveillance had already picked-up unusually heavy traffic by East Bloc freighters in the waters around Cuba in the months before.
Oleg Penkovsky also let them know that the Missiles were there.
And, if the missiles had remained undetected in Oct. 1962, Castro's big mouth might have given them away.