Islam's proselyting method was probably the best in the world to that point and maybe after. Death or taxes to the infidel, Muslims pay nothing. Such a great idea!
The Islamic empire declined because it didn't have any more infidels to tax
"At first, the Muhammadans came as liberators and brought relief to the inhabitants of an over-taxed and enslaved Roman world," writes Adams, but finally: "The Muslims had led the world right back to where it was before they had arrived on the scene. Only the names had changed. Muslim tax men ended up rivaling the worst of the Roman Empire. Perhaps this picture of Muslim tax chiefs, written centuries ago, best illustrates the end-product of their tax system: 'They were cruel rascals, inventors of a thousand injustices, arrogant and presumptuous... They were the scourges of their age, always with a causeless insult ready in their mouths. Their existence, passed exclusively in oppressing the people of their time, was a disgrace to humanity.'"
Charles Adams " A New History of Taxation"