Drones will be the next delivery system.
Are you presuming they haven’t been already? The only reason they may not have been used so far is their limited payload.
They probably already are being used. Wherever there’s something illegal that people want and are willing to pay serious money for inventive folks will find a way to deliver it. Fortunately, drones aren’t feasible for aerial transport of the human contraband the wall would be mainly directed against.