“its purely to allow a route for players to earn things by playing so they aren't forced to pledge for them but this is entirely optional.”
This is the very definition of pay to win. They will always try this "it's optional"-speech, but pay to win is if a player can buy what another player has to work for.
"but pay to win is if a player can buy what another player has to work for"
Both players had to work for it, it's just that one had to do real work, contributing to society, to earn the money to pay for that item, instead of doing "work" in a video games to earn said item.
How is Mantle an indicator of that? Of course their GPU is going to perform better on a low-level API like Mantle versus mid-level DirectX9/10/11. And Nvidia's will be getting the same benefit with DX12, it's why AMD is killing off Mantle, because DX12 does the same things, but for everyone.
Nvidia trimmed out a tonne of DP units in their 9xx series that allowed them to cram in much more gaming performance vs die-size and power usage than AMD chips. Nvidia won this generation, and their next GPU, Pascal is set to change up the GPU game a tonne as well. Massive layoffs for AMD on the CPU and GPU sides over the past few years, they don't have anywhere near the same resources for designing GPUs right now.
Why shouldn't Mantle or something like it be the standard? That's the better question, frankly. It's one that's being asked with the Vulkan API, which is the successor to both Mantle and OpenGL.
It's basically OpenGL 2. It's open source, it'll run on Linux, Mac, and Windows, and any cards designed to use it will have better performance just by the very nature of it.
Basically, supporting DirectX is playing into Microsoft buying the support of AAA developers just so that we're forced to use something that's objectively inferior.