Don't worry guys. Mantle 1.0 just get a new name and Khronos will release the specs.
This very much sounds like it will be the case.
AMD said:
Mantle’s definition of “open” must widen. It already has, in fact. This vital effort has replaced our intention to release a public Mantle SDK, and you will learn the facts on Thursday, March 5 at GDC 2015.
This simply speaks to the idea that they are now moving from an "open" project that has a publicly-available APK, to an open project that will be at least partially open to public development efforts. This jives strongly with the intent of OpenGL and what may be glNext, with Vulkan appearing to be the new name for that project.
Then there is this, which indicates AMD has been open to the new OpenGL drawing from the Mantle body of work, in whatever way Khronos Group saw fit.
The Thursday schedule at GDC as it relates to these topics is thus:
AMD's only talks that day, one is a TressFX 3.0 talk for Deus Ex Universe Projects, and the other is showcasing DirectX 12 efficiency and performance, with a Microsoft guest speaker
Valve is presenting glNext. One of the speakers? Johan Andersson, of EA and the Frostbite Engine team.
That last bit is pretty telling, IMHO. Frostbite has never been anything but DirectX and Mantle. They've never developed for OpenGL, and with already using TWO APIs, why would they tack on a third? Well, one possibility could be they are just replacing Mantle with glNext, end of story, but it could be that they are only even considering that because they've already laid the foundation and glNext is going to basically be Mantle 1.0 or at least draw very heavily from Mantle.
I feel confident in arguing that AMD was fully aware that Mantle would never be a long-term solution. They took what they knew of current DirectX 12 tech goals, and took what they learned and helped create for the Xbox One with Microsoft, and turned that into a PC API for their architecture, and took the time to include overhead reduction strategies and more "bare metal", which sounds like was not a big push of the original DirectX 12 development.
Regardless, they made an entrance as the THIRD API of importance on the PC: DirectX, OpenGL, and now Mantle. The world is hardly big enough for two. Three? I think AMD fully understood that Mantle would never "dethrone" OpenGL, frankly they could never even dream of competing with OpenGL in terms of developer mindshare or market presence.
That is why they offered up Mantle to Khronos Group last summer. Mantle was basically a public demonstration of a proof of concept, and AMD might just want Mantle to be pushed further to include more architectures and more platforms.
edit:
Heck, I think that Khronos Group has chosen "Vulkan" as the likely brand name for glNext is a pretty telling fact. Vulkan so heavily references the theme of Mantle that it seems perfectly intentional, and may strongly suggest how close the code and objective of Vulkan is to that of Mantle.
edit2:
Someone on Reddit said this, which is perfect as an image:
"If the Earth (Mantle) opens up you get a Volcano. Volkan -> OpenMantle."
It was previously mentioned that Volkan means Volcano in numerous languages.