Being a dev myself I would have to agree with the guys on the panel, after all they are arguablly the top three engine developers in the world.
They say that it would be a bad idea to have yet another API. Instead Microsoft and the Khronos group should take onboard what Mantle is doing to improve DX and GL.
This way we all benefit.
The problem is although Mantle can impact on DX in order shake MS out of its sleepy state regarding gaming, there will always be the trade-off of being more general but less performant versus being more restrictive and efficient.
In the days of Glide, where architectures were plenty, general won by a mile. But today, the PC space is divided in three different players (although only 2 are relevant for gaming). It is in the mobile space where the fragmentation exists.
Another issue is something I saw referred, but can't really coment on (for lack of knowledge): DX (and Open GL in a worse way) are based upon ideas and fundamentals brought from an old era and have become completely out of sync with todays game engine design rules and way of functioning. That a fundamental rewrite of the standard is needed.