That’s a pretty evocative question A.S. We could do some spectacular speculation on what kind of predictive luthery would keep your arm from dampening the sound of the guitar, though the solution is already there in the after market arm rest. Here’s a silly analogy: the design of a hand bell precludes damping by attaching a handle that doesn’t conduct resonance. When you hold the bell by the handle your hand doesn’t absorb the vibration and the body of the bell rings freely.
Same with a guitar, until you lay your arm on the resonant surface and damp the vibration with the non-resonent nature of your forearm. This happens more with larger bodied guitars that limit your ablity to keep your arm free of the body. It wasn’t always so. Guitars only needed to be big enough to be heard in the largest room in the house until around a dozen decades ago. These earlier designs were meant to avoid arm contact by allowing the player to comfortably reach over the body while playing. It turns out that relatively larger guitars like the Dreadnought are historically the anomaly. Over the millennia of stringed instrument design the Jumbo and D are recent adaptations made to produce enough volume to compete with banjos, mandolins and other instruments in an ensemble. With sophisticated microphones and pick up systems we now have other options that allow us to be heard.
Back to SCGC; we build guitars for appropriate durability with maximum resonance, sustain and complexity. We don’t compromise these qualities by trying to accommodate all possible dampening influences a player may introduce. On larger bodied guitars that don’t allow the player to comfortably keep their forearm off the body, arm-rests are an excellent aftermarket solution that saves us from adapting the build and messing with an already good thing. All the best, Richard