Targeting Mantle is not that much of an overhead because it:
1. Can reuse console code 'in whole or part (AMD)"
XboxOne supports DirectX at a high level and PS4 supports OpenGL at a high level, which facilitates porting this code to PC's, so I don't see the purpose of trying to port "part" of lower level code. And at a lower level, there are likely to be some differences in the console API's due to console-specific features added.
2. Already has a large GPU user base that can benefit from Mantle - the 7xxx cards.
That is incorrect. The percentage of PC gaming systems with GCN-equipped graphics is very very small compared to everything else out there. The majority of PC gaming systems with discrete graphics cards use NVIDIA graphics (while the rest use primarily non-GCN graphics).
3. Addresses both the GPU and CPU
Wrong again. The vast majority of PC systems with integrated graphics use Intel CPU's + Intel iGPU's. The vast majority of PC systems with discrete graphics use Intel CPU's + NVIDIA GPU's. The vast majority of Android/iOS/WinRT systems use Qualcomm/ImgTech/ARM/etc. CPU's + GPU's. Even many popular consoles today (and possibly in the future too) use non-AMD CPU's and non-GCN graphics such as Xbox360, PS3, 3dS, etc
4. GCN is a long term architecture. Mantle optimizations done today will work on AMD processors far into the future.
GCN is a generic name for AMD's current GPU architecture. GPU architectures evolve and change over time. Even according to AMD, there is no guarantee of backward compatibility with future lower level API's and current GCN hardware.
5. Mantle is designed from the start to be multi platform - Linux and Android can be easily accommodated.
That is not very relevant because Android has zero GCN-equipped hardware, iOS has zero GCN-equipped hardware, WinRT has zero GCN-equipped hardware, and even Linux support from AMD has been dubious at best.
With most of the major developers (and engines) on board and cross platform future proofing Mantle is looking very strong.
Actually the long-term prospects for a very low level IHV-specific graphics API such as Mantle are very dim when you consider the need for game developers to increasingly target more and more [ultra mobile] platforms and more and more [ultra mobile] graphics vendors.