Islam appears to have gone through a long slow decline from the end of the Islamic Golden age and has been slow to embrace modernity. The Islamic Golden age appears to have been (at least in part) the catalyst for the European Renaissance. The European renaissance has enabled the Gospel to be spread through all the nations (Matthew 24:14) coinciding with the Jewish diaspora ending (Luke 21:24) in the 19th century.
From what I understand Adrian, is that the decline initiated on the passing of Muhammad at the overriding of Ali as the rightfully appointed successor.
Yet the influence of Muhammad did bring about an age of progress, which in turn influenced other cultures. But this progress was always to be overshadowed by the negative influence initiated by that rejection of Ali. The Bible to me says this in the metaphor of the dead bodies laying in the streets.
Muhammad also foresaw this event, as you would be aware, in a shrouded prophecy of the invocation of, "there is no God, but God". The negative coming before the positive.
Thanks for the thread, lots to learn
Peace be with you.