Microsoft, what challenge?
Quote: "to challenge the Rift with HoloLens".
Rift is V.R. HoloLens is O.R. not the same market, so how exactly is HoloLens going to challenge the Rift (or any other VR headset) for market space?
So if you want fully immersive VR, you'll buy a Rift (or other VR headset), if you want OR, you get a HoloLens (or other OR headset).
Not saying you won't get games running on the HoloLens (I can image some would be quite cool), but they won't be the same sort of games you'd have running under VR.
I could image the Besiege game being quite good to play in HoloLens, if they can get enough performance into the device. (HoloLens is a standalone device, so self contained, so your going to be CPU/GPU/Memory/Storage limited, unlike the Rift).