Actully they had a tall basketball player sit in one yesterday at the LA Auto show and he stated he felt comfortable in the seat.
In the spot where I saw this, they showed the car broken down, and Mercedes has put a lot reinforcement all the way around. It’s not the death trap some would think this small car to be.
If they reinforce the outside, that just means the occupants get splattered on the inside in a serious crash.
They can't change the laws of physics. If two objects collide, the one with the least mass will take the brunt of the impact. It will experience the greater acceleration / deceleration.
They can lessen the impact felt by the passengers by spreading out the force over a longer period of time. This is done through crumple zones (which there aren't room for much of in such a tiny car) and air bags. Having to build such a solid frame to protect the passengers from objects intruding into the passenger compartment actually works against them when trying to lessen the overall felt impact.
If they allow it to crumple more to reduce the overall felt impact, they risk having the passengers squashed.
The simple fact is that if you're in a tiny, lightweight vehicle, you are going to take the brunt of the impact in a collision.
A rigid frame and airbags is probably the best way to minimize the effects on the passengers, but you are still much safer in a more substantial vehicle.