C'mon, "no such thing like "maximize performance""? If NVidia and Intel hardware run like crap on Windows 10, then Windows 10 fails, period. No way in hell am I buying that Microsoft isn't concerned with maximizing how Windows 10 and DX12 perform on all platforms. Maximizing performance is the entire reason to create yet another Direct X system, much less one that radically departs from the existing software.
Again, no one (or almost no one) is claiming that DX12 (or the D3D part of it) used the actual Mantle code as its starting point, just its concepts. No way in hell are the two so conceptually similar as to use parts of the documentation word for word by coincidence. I'd also bet that some (maybe a lot) of the data structures are at the least function-for-function identical. AMD has a vested interest in its hardware running well within DX12 (especially with Microsoft taking over development and maintenance, which saves AMD a huge amount of money that would be spent on Mantle) and Microsoft has a vested interest in developing the best product in the least amount of programming time.
An API is not a magic tool that maximize hardware performance. You - developer - have to maximize your application performance through that tool. APIs like LibGCM, Mantle, Metal, Direct3D 12 and Vulkan are meant to reduce the impact of the API runtime on your application, to reduce the driver background workloads and give you less abstraction as possible to better control the hardware.
Just because you make an #include <d3d12.h> doesn't mean you are going to receive a free performance boost into my application. You are still able to make crap like like
this, probably if you are a bad developer you will produce more "things" like that with a low level API.
AMD and DICE did not invented the wheel with Mantle. Mantle simply came out first, it was designed for a limited range of hardware and it was designed to run against WDDM 1.x. It was simple to put it out first. Mantle has a huge credit: it showed first how much limited were Direct3D and OpenGL with current modern hardware and it showed a part of the solution. No-one can deny that.
However, Direct3D 12 is meant to run with a larger range of hardware (which is not an easy challenge), it's meant to run with a new driver model (which required a lot of work by Microsoft), moreover it's meant to work with older and legacy graphics Windows API too (even if that's was not a primary goal though) and add set of new capabilities too (like SM 5.1), and there are the graphics tools. Build Direct3D 12 required a lot of work compared to Mantle. It is obvious that AMD and Dice where able to put it out first.
Intel and NVIDIA hardware do not have any issue caused by WDDM 2.0 and the D3D12 structure. I do not understand where such concerns come from. If we speak about rasterization, Maxwell 2.0 is the most advanced GPU currently on sale, no-one can negate that.
Finally you cannot directly compare the amount of money AMD invested into Mantle to the money Microsoft and it's partners invested into D3D12. There is at least one order of magnitude in difference, maybe two orders of magnitude. AMD itself stated they didn't invest a huge amount of money into mantle. If you wonna find someone that invested a lot of money into mantle it's called EA.