I agree, you are safe.
What's interesting, is I didn't hear ANYTHING about a "DX12" API until Mantle came out. All of a sudden people are like "Mantle rocks and is closer to metal. blah. blah." which is true once the bugs are ironed out.
Then all of a sudden Microsoft has a new API coming soonish? Really?
To be fair, it MAY have been in the works anyway as the XBOX ONE supporting DX11 has been known for a while so modifying the closer-to-metal XB1 DirectX API wouldn't be too hard.
The problem that crops up is GPU compatibility though. You have to draw a line and say "these old cards don't work" such as pre-GTX600 and pre-HD7000.
As said though, this is basically academic as GAME DEVS can't afford to come out with games which support only the latest cards. Just look at ANY game on the market today for the minimum specs.