I'm not arguing against crude explosives, but for your 200 years and technological advancement statement, the Empire basically crumbled. You've heard of the Dark Ages following the Roman Empire's collapse, correct? Time does not equate to technological advancement. The opposite actually happened in western Europe after the Roman Empire collapsed, so why would the collapse of the Cyrodiilic Empire necessarily be much different?
so many other empires have collapsed... you can't really mean this. after the fall of the Roman Empire, after the crusades, There was rejection of classical learning (later revived in the Renaissance) and a large increase in devout, unquestioning faith in the Bible. The Clergy possessed libraries of books which had all the knowledge, and they burned all the books that didn't agree with Christianity. Thats mostly why all that knowledge was lost.
fall of Empire=/=fall of knowledge
EDIT: also, It is pretty much a fact that war brings on innovation especially in things like explosives.