No one in their right mind would build (NEW) product today exclusively on DirectX or any vendor locked product. With OGL running on Mac/Linux/Windows/iOS(es)/Android(es), it's a no brainer to put your support there. We just need more companies like id software that put their muscle behind it.
I'm programming my own game now, and completely ignoring DX support. What advantage does it give me if I can get acceptable performance out of OGL? Nothing, just another target that has to work correctly. I want my game to run on your Mac, your Windows machine, and your Ubuntu machine. Don't you?
If I were Valve, I would simply start supporting OGL-only on my own games. Force AMD/NV/Intel to support it better on Windows in their drivers- helps get the ball rolling for SteamOS as well.
That said, if I were Valve or DICE, I'd welcome Mantle if the performance advantages were useful needed. It's apparent they were, if devs are asking for lower level APIs like Mantle. AMD is just the company in the best position for such a thing, and no one else is. I'm definitely more interested in picking up AMD hardware across the board going forward, namely HSA class APUs and onward.