You're right, AR can be much more than just a heads up display. However, a direct brain interface is even farther away than mainstream AR, which is already at least 5-10 years away, and unless it can be done without directly implanting a device into your brain, 99.9% of people aren't going to use it.

I'm currently skeptical on how feasible a direct brain interface is. It's most likely going to be very invasive, and who knows if it will be better than just really good eye tracking and hand gesture recognition for navigating interfaces.