AMD indicated from the beginning they wanted it to become the universal API.
Opening stuff up formally also takes time. So it all was going towards Vulkan in one form or another and no one was forcing MS to push DX12 NIH while this was happening.
And counter to your point, despite Mantle being "proprietary", MS directly used it to create DX12 (same as Vulkan used it), so AMD clearly didn't have any complaints about that.
Over time the work was obviously done for Mantle → Gl Next → Vulkan. And that's becasue AMD were positive about this idea. Their initial presentation of Mantle was in that vein, i.e. to kickstart the progress of the common API.
MS just decided to do that whole thing for their NIH in parallel using parts of Mantle practically verbatim. It wouldn't have been possible without AMD basically allowing it.
See: https://x.com/renderpipeline/status/581086347450007553
I.e. I don't see any reason here for MS not to collaborate on Vulkan instead, besides the usual lock-in approach.
> AMD indicated from the beginning they wanted it to become the universal API.
Was it an indication or was there any actual work done? Such as supporting anything else but AMD cards for example, inviting others to collaborate etc.?
> despite Mantle being "proprietary", MS directly used it to create DX12
I can't remember the term for it: what do you call when a single company develops something with little to no external input and collaboration, even if it's sorta kinda open?
As for "NIH"... Microsoft has/had a much bigger investment and interest in new gaming APIs than the few AMD cards that Mantle supported. And they already had their own platform, their own APIs etc. Makes sense for them to move forward without waiting for anyone