Actually, it was written while he was alive throughout his life and muslims believe that it was the word of god sent to him by archangel Gabriel. Actually the time after his death and for 500 years afterwards is referred to as the golden age, which technically had them surpassing the europeans in fields on science and technology. The decline occurred after the Mongolian invasion in the 1200s, which destroyed everything. The mongols would eventually assimilate and become the many muslim turkic people in the middle east and also gave rise to the ottoman empire.
“Actually the time after his death and for 500 years afterwards is referred to as the golden age, which technically had them surpassing the europeans in fields on science and technology.”
Islam doesn’t encourage creative endeavors, so the technical and scientific advancements came from the peoples they subjugated.
The Assyrians are a good example. As a people, they still exist today.
Brief History of Assyrians by Peter BetBasoo
http://www.aina.org/brief.html