John Merrells makes the point that DIX is an “ethernet” for identity protocols. I would say it is akin to TCP/IP, but I agree. In its initial incarnation it attempts to solve no more than is required for exchange of identity information. It is a layer of the solution that enables the higher layers.
There are good reasons to follow the network stack model of layering functionality - we are having these high level conversations about identity because the underlying supporting infrastructure was created in layers. Those layers became commoditized at different times allowing more and more innovation to occur at the higher layers. Trying to solve the whole identity problem for the whole internet in one giant step is never going to happen - it would be like inventing “the web” as we know it today in one fell swoop.

Johannes Ernst showed a picture at the IIW that supports that argument nicely.