I’m sure those who have damaged their spines have never intended to do it.
Does seem plausible.
The risk of crashing is always there; but the risk of a crash damaging your cervical spine is very low. And also, not limited to downhill- just naturally the risk is higher as the risk of big crashes gets higher. But it never actually gets very high.
Real-world proof of the effectiveness of the things is always going to be incredibly hard to come by (though, on the plus side, that’s just because you need a massive sample of riders to get a significant number of spinal injuries, never mind preventable c-spine injuries, let alone a comparison of with-and-without, ruling out user error. But the theory is certainly good.
Seeing people wearing cheap, heavy full face helmets and leatts seems odd to me though, lighter helmets reduce the risks involved too. Likewise people wearing them but riding shagged-out bikes or attempting things they just don’t have the skill to do.