While HTTP2 is a layering violation incarnate, apparently properly layered solution is undeployable. Perfect is the enemy of good.
If there's a multiplexing problem to be solved on the Internet, it more-or-less must be solved above TCP, no matter what the "layering" guidelines are.
Meanwhile: I'm still not clear on why these "layers" exist, or need to be dignified. The whole idea of a "layer" of complicated functionality is in tension with the End To End Argument: if there's a debate about how something should be implemented, that thing should be implemented as close to the endpoints and as close to the application as possible.
The really frustrating thing is that Google is already working on QUIC, which is probably the best approach with a realistic chance of success. It removes TCP from the stack and provides a general purpose transport layer rather than one which is tightly coupled to HTTP. Unfortunately Google decided to push SPDY with it's own kludged up transport rather than taking the time to do it right with SPDY over QUIC.