VR gaming on the pc will be no more niche than pc gaming itself. VR puts you into the game environment. AR puts things into your real world environment. They aren't the same experiences. VR headsets are already selling at more than niche on more than one platform. Frankly from a gamers perspective (playing since Pong) I see nothing in AR that threatens the VR gaming experience on the horizon.
And any VR headset will be capable of AR with the addition of some some stereoscopic camera's on the front.
AR isn't going to kill VR, and VR isn't going to kill AR, It's going to be two different use cases for the same device, sooner rather than later there will more than likely be one kind of device that does both.
I think I already saw someone making an addon camera module for the CV1 so it could do just that.
The biggest problem with AR is that the screens arent acutally perfectly translucent they are just screens, so you can't wear it and see your screen in RL resolutions and with added bits from the AR system.
Translucent OLED might be able to achieve this, but I suspect they still very rare and not in any form of production.