Amir Giles said:This video explains what I've been trying to say, that the shape of the lumbar vertebrae allow far more extension/flexion than the thoracic vertebrae, but the thoracic spine allows more rotation:
I think that there's a bit of confusion around goals and terminology here. The goal here is to build a strong arch, not to bend backwards and put your feet between your head. The latter is more of a goal for yoga, where mobility is the primary goal. A strong arch with proper muscle activation and full body tension is a foundation that can be built upon towards later gymnastic skills. Imagine doing a back handspring in the hyperextended position that you showed, bad things would happen.
An arch that focuses on the lower back may allow for more ROM, but it's extremely unstable. People probably naturally bend at the lower back because it comes more naturally and it's easier, as you've pointed out. But in terms of gymnastics strength, and even engineering, a strong arch is rigid equally across the entire arc. And since the lower back does tend to bend quite easily, proper form should focuses on bending at the hips and upper back, and the lower back will follow along naturally. If you focus on tensing the lower back, you're creating more of a V shape, as well as a point of weakness, not a strong continuous arch.
Back to the original point of stability and flexion in the spine. It seems to me that the lumbar spine should be stabilized because it tends to overextend, and the thoracic spine should be mobilized because it doesn't tend to bend. While the pictures you showed may demonstrate the natural tendency of the spine to bend, in terms of balancing strength and mobility, it is not ideal.