This thread is going nowhere because you don't have a sufficiently long perspective on the matter.
During the European Middle Ages armor for the knight improved mightily until about 1200 the knights were virtually impregnable. But later knights were killed again on the battlefield and the price paid in mobility for higher protection was too steep. With the replacement of longbow and arquebus by musket before 1600 body armor started to disappear entirely.
A delightful book on this matter was written by Tom Wintringham in 1943 - Weapons and Tactics from Troy to Stalingrad., Houghton Mifflin, Boston, USA 1943, republished 1973 with Col. John Blashford-Snell ISBN 0-14-021522-0
( I have the Penguin edition with the addition of the thoughts of Col. Blashford-Snell - a truly vast difference in quality of thought between the two parts of the book ). See also Tom Wintringham - Wikipedia, the free encyclopedia.