Follow some of the top-end OpenGL developers on Twitter sometime; you'll change your tune quickly.

There's a significant amount of overhead in the existing OpenGL API, which currently can only be mitigated somewhat through vendor-specific extensions. But extensions alone aren't sufficient to address some of the serious issues that the OpenGL API has today.

OpenGL forces you to maintain a tremendous amount of state information that significantly increases overhead.

John Carmack and many others have explicitly mentioned this in past discussions.

While I'm horrified at the idea of revisiting the days of 3DFX glide and installing my own client ICD into a game driectory, if Mantle drives the OpenGL ARB to start seriously addressing those issues and increases competition, then I'm all for it.