Kevlar doesn't really break down. The newer fabrics like Spectra might and Zylon definately does. Some reasons for the 5 year warrenty. First, they want you to buy new vests otherwise they don't get more money, and infact lose some with repairs. Second, if the vest isn't properly cared for the effectiveness can be reduced, exposure to chemicals, sunlight, bleach etc. The carrier, and inserts can have their covering material worn or damaged exposing the Kevlar to damage itself. It can fray if edges are exposed. And new materials are coming out that can increase protection or reduce weight. Well the last hasn't really happened in a while. Not really anything better than the 4th generation Kevlar 129. Twaron is also good as it is almost the same thing as Kevlar but made by a different manufacturer. Spectra seems to be a good product too.