> What unsolved hurdle do you see?
Drawing an opaque, dark line. Almost all these screens are adding light to an existing scene. The only way they can make black is to just not project light, which means you need to have a dark background to actually show dark.
Ever view a projector outside in a bright sunny day?
Could you expand on this? It's seems absolutely certain, with products like nreal Air existing: https://www.nreal.ai/air
What unsolved hurdle do you see? Heck, there were even prototypes of laser retinal projection at CES!