Ahem, the symbolic representative of all WWI servicemen is the soldier known but to God who is interred in The Tomb of the Unknowns at Arlington.
However, I think that as the last surviving Doughboy, Mr Buckles deserves the honor of laying in state as a means of tying current generations to our past.
He is not the symbolic representation, but he is symbolically representative. His funeral, and his final resting place, will make an unspoken statement about our recognition of all vets, especially those who served during WWI.