But wouldn't these just keep using DirectX, OpenGL, Unreal Engine 3, CryEngine 3, Flash, Silverlight or whatever?
With using engine platforms (CE3, UE3, etc.) you still have make sure each rendering path works with your game correctly. And if you modify the engine in anyway, most devs do I assume, then you may end up having to code your own rendering paths anyway.
With regards to DirectX and OGL, would IHVs really want to take on the task of providing yet another API that's closer to the metal, bugfix an additional API, optimize an additional API, convince Microsoft that they should allow an API closer to the metal which may or may not jeopardize system stability, etc... Intel aleardy can't keep up with just one API to worry about. AMD while generally just as robust in DirectX has traditionally trailed in OGL, and that's just 2 APIs.
All of which means you will quite likely have developement gradually favoring one IHV over all others.
For example, lets say IHV X can only keep up with IHV Y in one API, gets close in another API, and just gives up on the 3rd. If we use DX and CTM (Close to the Metal), that means they give up competing in the professional market where OGL is far more relevant. And means they'll trail in performance/features in either DX or CTM. Which means end users will gradually tend to favor the IHV with more resources. Which means less resources for the other. Which then means less ability to keep up with 2 APIs, etc.
And I only mention CTM as an API as I'd imagine at least Nvidia and AMD would be reluctant to divulge all information regarding their hardware for fear it could give their competitors an advantage.
IMO, it's best to just keep going as we have been. Software developers pushing MS to allow more access to hardware. Meanwhile, MS evolving DX to better meet the demands of Software devs and pushing IHVs to include features Software developers are demanding while balancing to an extent what each IHV is currently capable of delivering for the next generation. If MS can't keep IHV hardware similar if not the same in terms of capabilities then the PC market will most definitely shrink as we go back to a time when hardware could have greatly disparate feature lists (80's to mid/late 90's).
Remember how things were with fledgling 3D acceleration? 3D acclerated game for Matrox couldn't run on S3 or ATI. And same thing going other ways. So for example, you had to create a specific version of Mechwarrior 3D for Matrox, ATI, S3, and whoever else had a card. It was a bloody mess. Then you had Rendition and 3dfx entering with their own ways of doing things and again you had to code specifically for each one. And eventually the only 3D accelerated games being made were for 3dfx with everyone else being ignored until OGL and D3D matured. At which point Nvidia entered with Riva 128 (D3D), Intel with the i740 (D3D), ATI switched to D3D and started abandoning any proprietary efforts, etc. And it's only at that point that 3D games started being made that could run on all vendors cards.
Regards,
SB