The point is not simply that someone would attack the Canal, but that we can prevent someone from transiting through it.
We do have airplanes. And Guantanamo allows us (1) to launch aircraft from Guantanamo to anywhere in Latin America with a shorter flight time than combat aircraft originating in the United States would have and (2) to replenish aircraft carriers operating in the Caribbean in less time than it would take to replenish them from the United States. Guantanamo acts both as an aircraft carrier that cannot be sunk and as a logistics base for carriers operating throughout the Caribbean. In the last 30 years, the base has been invaluable in actions against Granada and Panama.
Seriously--you cannot be so ignorant of naval matters to believe that forward bases are of no military significance. By that theory, we should abandon our naval bases in Bahrain and Japan, and the Royal Navy should abandon its bases in Oman and Diego Garcia.
I mean, since we have airplanes now, what's the point in cutting hours off their foreseeable flight times? Right?