You answered it in your question. WWI is known as the death of the old world for a reason. Where all nations truly had to fight tooth and nail with all resources going to the war.

Also a feathered helm and shiny breast plate would have been a death sentence, just ask the French cavalry from WWI

Essentially, they serve no practical purpose on the battlefield.

This may be slighty off the rails but one of my favorite examples is the famous myth of the horned helmet of viking warriors. Scandanavians used this cerimonially during their bronze age, it was unfit for battle (you don't want to give an enemy something to grab on to) and was not used during the viking age

It was the difference between being recognized and therefore not shot at, to more accurate weapons being able to hit you from 200 yards. In WW1, there was plenty of argument among the French to keep the red trousers out of pride, versus wear grey or dark green so they wouldn't be such an easy target. Any decoration on the field is the same way.