>>64132235
I think you vastly underestimate the amount of padding that's going to be under one of these, nevermind another helmet. The metal helmet will dissipate blows over large area and the way greathelms are made specifically won't really let your head move to the degree where you'd break your neck easily from a blow.
OP's question rings very true here. Armour is like any other form of protection - if someone has physical access to it and arbitrary amount of time and resources then it will be broken.