Modern safety standards make it so that the seatbelt locks in a crash and limits your longitudinal inertia.

That’s what I was trying to say.

I’ve now realised that I’ve explained myself poor. To reiterate;

Seatbelts reduce whiplash, so does correct posture. Poor posture inherently leads to a loosed seatbelt because it extends the range between you and your seat.

A lot of people consider a crash which lurches you forward, but if you get rear ended, the difference which matters will be your posture. If your head and neck are cushioned, you’re going to be much better off.

Fair point about the broken spines. It’s not hard to imagine how much worse things could be without correctly fitted headrests and seatbelts.