What are your thoughts?
Direct3D in high-end PC gaming shouldn't die any time soon because NVidia GPUs won't support Mantle, But if the performance improvements on AMD side are significant (eg: 9x more draw calls), consumers will shift towards AMD products so NVidia will be "forced" to release a Mantle alternative.
Only developers that really want to push the visuals forward are likely to use it though, small indie teams/studios will probably stick with DirectX (/OpenGL) because it's fast enough and easier (?!) to code.