MS could never kill Steam, they are too entrenched. Too many people have Steam product they need access to. It's just not just about future sales. Anyone who has ever bought anything from Steam has a horse in this race.
Steam has a virtual monopoly in digital game sales at this point so let's be careful with using that word. Even with Origin, GOG, Amazon and impulse, Steam still moves more product than all of them combined. Anyways, I don't believe for a second that MS is even thinking about trying to kill Steam. The MS App store is aiming for a completely different demographic. They want the average joe nongamer that they can shovel fluff to just like Apple has with iOS.