You can't take any sort of evolution or natural selection into account with TES. All things, especially dragons, were created and divinely inspired. It can't really be determined the exact properties of dragon materials because they may not even adhere to the structures created by living things as we understand them. If it's any hint as to what a dragon, in TES, might be like, Alduin and Paarthurnax are, in nordic tradition, the descendants of the Aedric deity Akatosh. As well, Akatosh is depicted as a dragon (remember the main quest line of Oblivion). We know that, for the playablility of dragon hunting in Skyrim, that dragons are susceptible to weapons but that does not necessarily mean that they are not much more powerful in lore. If you still think that major changes need to be made to the dragon armor system then why not separate the materials and armor by dragon color classification instead. It is established that different types of dragons have varying levels of power and this would not conflict with TES lore.