Honestly, it's a good thing for all PC gamers as a whole, but probably not a great thing for AMD going forward. There's a part of me that's a bit sad they didn't develop Mantle further specifically as an AMD only feature. It was one that would have set them apart at least from their competitor. That being said, I do find it a bit troubling that you'll see posters come in here cheering the death of Mantle rocking 980's in their sigs. Will they be so happy when those same mid-range/high end cards start selling for $100-200 more with no price drops in sight if AMD disappears.
Well AMD had to of known in the long-term Mantle could not be sustained as an AMD-only initiative. They learned a lot working on the Xbox One with Microsoft and found a way to bring back a low-level vendor-specific API. This has, really, always been possible, and used to be the norm. It wasn't on anybody's radar to develop such because vendor-specific APIs are a bad thing for the market in the long run.
I fully believe AMD had no intention of keeping Mantle to themselves. It removes a little bit of a unique performance lead for their products only, but they also recognize they had a small marketshare and couldn't afford to be pushy with developers the same way Nvidia can. When previous manufacturers had such a lead, it was the same way.
A Mantle that remained indefinitely as a vendor-specific API would not be popular for long these days, because while it brought huge benefits to the market, it helped splinter developer resources and if it was truly successful as a vendor-specific API, it could splinter game compatibility and risk going back to where huge performance advantages (and disadvantages) with major IQ differences would be the norm, and determined based solely on which product you chose. You get that in a very very limited sense these days, with a library of additional features for one camp or the other, but not at the API level.
If Mantle was very successful, and failed to spur Microsoft to develop a similar vendor-neutral version, I don't doubt that in time we would have seen Nvidia try to push something of their own.
Of course, all of what happened is very much the natural result of Mantle coming onto the scene. Perhaps the glNext initiative would not have occurred in some reality, but one way or the other, somebody, be it the Khronos Group or Microsoft, would invest time and resources to develop a similar Mantle-like vendor neutral API. And, as we see, that is exactly what happened.
Except in this reality, AMD saw how beneficial it would be to actively help get high-quality vendor-neutral APIs on to the scene, two of them, in fact. AMD contributed to Direct3D 12, as did Nvidia and a host of other parties. Likewise with Vulkan, all of the major players participated and helped shape the API.
I'm not arguing that all of the Nvidia fanboys (again, count me as one if I must be labeled, I'm still worried about not having Nvidia for GameWorks games and wanted the 980 in SLI had it not cost so much) should bow down and praise AMD like it was a god, but we humans should be mature enough to recognize good things when and where they happen, no matter who provides.
Nvidia is just as much to thank for some DX12 features, I'm sure. And it is likely not using Mantle code whatsoever but there is assuredly AMD touches that produce the same effect.
Whatever one believes Vulkan is, and regardless if one recognizes it as a proper successor to Mantle, it doesn't matter. The fact of the matter is Vulkan and DX12 are almost here, and AMD directly contributed and pushed these results. This is good for everyone.