You appear intent on blaming the Islamic kingdoms for the downfall of the Eastern Roman Empire. It's not at all clear that they would have flourished in the absence of Islam. It's likely they would have been swept aside by the Mongols.
Even otherwise, you realise that the body blow to the ERE wasn't struck any Muslim army. Constantinople was captured and looted by Crusaders in 1204, apparently on the Fourth Crusade. Though the Greeks recovered Constantinople a few decades later, it was a shadow of it's past self, left with the city, north Western Anatolia and parts of the Balkans. The Ottomans ended the empire in 1453.
Even otherwise, you realise that the body blow to the ERE wasn't struck any Muslim army. Constantinople was captured and looted by Crusaders in 1204, apparently on the Fourth Crusade. Though the Greeks recovered Constantinople a few decades later, it was a shadow of it's past self, left with the city, north Western Anatolia and parts of the Balkans. The Ottomans ended the empire in 1453.