The biggest thing restraining it is the ergonomics and form factor. No one wants face PCs. Imagine how women, who are half the population, and who spend 0.5T dollars a year on hair and makeup products, are going to deal with smashing a 1.5 pound PC into their faces and held in place with straps around their head and hair. SJ would have smothered that thing in the crib the second his team had no answer for how a product that excludes half the population at the get go could possibly be successful. Can you imagine iPhone shipping in a form factor that required women redo their hair and makeup after every single use?