We have lost friends over not allowing them to ride unrestrained in our car.
I did reluctantly give a ride to a woman who refused to use the shoulder belt once (she claimed it made her "claustrophobic" and acted like she would DIE with it on) because it was extenuating circumstances, but I made it very clear that she would not ride in my car again, and at least she had the lap belt to keep her more or less in her seat.
Do we know how fast those cars in the video would be going, approximately? I showed this to a family member who insisted they didn't need to wear their seatbelt in the neighborhood. The speed limit on the road we were going to be on is 35, but of course people go faster.
I would approach them with basic math. force = (speed x weight) ÷ ride-down. So if they weigh 150 lbs and they hit a telephone poll at 35 mph, that is 5250 lbs of force (over 2.5 TONS) divided by whatever ride-down the crumple zone buys you. Double that if you hit another car going 35 mph in the other direction (in fact, that would probably be considered an "unsurvivable" accident if neither car was braking/slowing at the point of impact). Even if the car takes 75% of the force in the telephone pole scenario, you're left hitting the steering wheel with around 1000 lbs of force if you don't have your seat belt on. The seat belt stretches and provides extra ride-down, further dividing how much actual force you get. Hitting a telephone pole at 35 mph, even with a seat belt, is a very very serious accident that's likely to produce significant injuries and it would be a miracle to survive it at all without a seat belt.
People think 35 mph is "slow" but that's because those people haven't been in a 35 mph crash.