We tend to forget that before Byzantium fell to what became the Ottoman Turks, it had been badly hurt in many conflicts with Western Christian military. It was probably already doomed as the Ottomans conquered both Asia Minor and the Balkans before the final fall and thus cut-off from land-based support. Western Christianity later paid a heavy price in the Ottoman’s attacks that reached the gates of Vienna several times.
That is the real start of the decline of Islam - they lived for centuries off being the middle men in trade between the east and the west.
The Byzantines were able to deal with trade direct to China for a time via their Jewish allies, the Khazars - and in a famous episode of Byzantine craftiness the Byzantines smuggled silk worms out of China making them the sole source outside of China for the stuff.
Islam declined the way it did because the Europeans bypassed them and they could no longer tax the trade between the east and west. The Muslims never recovered from their victory.
This is my opinion only and it is sort of worth pondering that for the Muslim world, capturing Constantinople probably destroyed Islam in the long run.