Steam isn't going any where and I don't think MS thinks that either. In Phil Spencer's presentation he actually launches the game on Steam and talks about how well Steam works on Windows 10.
Steam is Steam and will still be a leading platform for PC games. The Windows 10/Xbox thing has little bearing on it. I'm a console gamer, but if I bought a PC to game on, it would be for PC games like Eve Online or SWTOR.







