and proprietary apis are not the answer to positive developer experience. time will show whether insisting on your own world-incompatible graphics apis will lead to developers writing custom apps for your platform, or whether they will use whatever lets them release on many platforms at once.
Indie titles shipped using OpenGL ES on PS3 and indies got Sony to eventually release an OpenGL wrapper for PS4 as well. If you want decent portability and all your game does is draw alpha blended sprites with simple shaders it could be a win. Questionable how much better that is than either making your game with a portable engine like most indies are doing, or using gnm directly.
mobile dev seems a lot more (for want of a better word) grassroots than professional game dev. lower barrier to entry, and a larger set of people who have a good idea they feel they can implement. also phones will probably bring up a lot of uses for a good high-performance graphics api that are not game-related, as people think of more and more things they would like to do with the computer in their pockets.
