Maybe you've seen this, but it was new to me - a three-point seat belt in a commercial aircraft.
It was like the older three-pointers in cars, with the shoulder portion hooking in to the lap buckle, so you didn't have to wear the shoulder strap if you didn't want to.
Now, if you auger in to a cornfield at 700 km/h, I don't know that the additional safety provided by this shoulder belt would make much difference.
Frankly, no belt would make any difference in that instance. Belts on planes really only help much in ground 'events' or to reduce injuries due to in-air turbulence.
But I guess every little bit helps.
What's next - air bags?