I think you are placing far too much importance on the impact of consoles in the PC space. Just because Xbox One has a similar API doesn't automatically mean every dev will start lazily porting Mantle API games to the PC. They'll maximize profits and that means optimizing for DX 11, not Mantle.
What you're overlooking is the fact that this is a low-level API that's written for the hardware contained in the XBox One, the PS4 and GCN equipped PC video cards. Low-level programming means you'll get higher performance out a given piece of hardware than going through DX.
And if developers decide to use Mantle for their console games, they can easily copy it over to GCN equipped PC's. Those GCN PC's will then enjoy the performance benefits that come from low-level programming over a high-level API such as DX.
NVIDIA + Intel marketshare is far too important and lucrative to give up for a company that has been on the losing end. Also keep in mind, developers don't dictate what goes into a game, its the publisher most of the time because they're the ones funding it and they'll always push for the most widely used standard, not some Glide wannabe API. If anything, I foresee Mantle being used by Indie developers and kickstarter projects.
I'm sure DX will continue to be around for quite some time. It's not going to just roll over and disappear because AMD created Mantle. But being able to create low-level programming for the XBox One, the PS4 and GCN PC's simultaneously will be a huge boon to developers.
Will they continue to make DX paths for PCs? Of course. But they simply won't be as efficient as their Mantle counterparts. GCN equipped PCs will end up with higher framerates by running Mantle than they would by running a DX path.
Mantle doesn't hurt Nvidia owners. It simply gives a boost to AMD's GCN equipped customers.