Cameras can fail, the communication links between the camera and whatever serves video stream can fail, the displays themselves can fail, power to the displays can fail, etc.
If you do a simple HUD overlaid on top of the conventional glass window, at least you are not completely screwed even if there is a compounded instrumentation failure as long as you can still get visual cues somewhere such as the moon, stars, city lights, runway and slope guidance beacons when landing, etc.
As long as you still have some degree of flight control left, line-of-sight, skills and luck, you can land a plane even with extensive instrumentation failures - there have been many seemingly impossible yet successful landings.
Having cockpit displays/HUDs to aggregate, complement and supplement existing instruments is fine but for completely replacing direct line-of-sight as a backup? I don't think so.