Not sure this makes sense pedagogically. I would challenge you to try this in a classroom of average middle schoolers and see how it goes. I don't think you grasp the difficulties here.
But the length of the real proof is beside the point. We teach them about 1 and 2 before going to 1+1=2. And when we do go to 1+1=2, we show it to them using objects and counting. We don't simply tell "here's the function for addition, just put numbers into to to solve problems".
And that if you instead start with squares and rectangles and built up to it, it'll be less daunting.
But my point wasn't about this one formula specifically, but about the approach in general.