I think Gerard is right but with a significant caveat. First, no matter what you do, you still need to have fork blades that are long enough to give good tire clearance, depending on the purpose of the frame. If the disc frame is essentially the same size (regarding the rider contact points) as the rim brake frame, yes, a disc brake bike could allow one to virtually eliminate the crown area of the fork. BUT that frontal area is not truly eliminated, instead it is replaced with a longer head tube. So then, the question is, is this replacement of one thing for another thing a benefit aerodynamically, or a wash, or an aero disadvantage? The answer to that is, 'it depends'. It depends on the size and shape of what was there previously, and the size and shape of what you replace it with.

If you look at what a Trek SC does with its front brake, you can get of an idea of what can be done with a pretty close to optimal (aero-wise) rim brake system. Can an optimal disc system beat this aerodynamically (after factoring in the impact of a disc hub, the braking rotor, the brake caliper mech, and the extra spokes needed)? Maybe, but we haven't seen it yet.

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