I have the "wheel-assist" you describe - the Yamaha NaviOne. it works incredibly well, and without it I would need a full power chair since my arms are fragile.

Auto brakes as you describe would be really annoying since it would make the alternate-hand technique impossible, and most wheelchair users stop touching the rims between strokes. (I use the handrim grip favored by quadriplegics because of my arms, so I so keep my hands on the rims at all times, but it's not ideal.)

I agree that disability aids are quite overpriced and lackluster in general, but I love my Yamaha wheels.