It's not about "lower". We can, and do, make massive , non-backwards-compatible changes in layers 1 and 2 every few years and no one is the wiser. Layer 3 is s

It's not about "lower". We can, and do, make massive , non-backwards-compatible changes in layers 1 and 2 every few years and no one is the wiser. Layer 3 is special. It makes that flexibility in all the other layers possible, but keeps none for itself.